A solution to MDF Expansion
There's a real problem spraying or painting MDF - it expands over time as it soaks up the moisture from the paint and the surrounding air. The net effect, as I'm sure your aware if you've tried this, is that joints reappear after a period of time. This could be a few hours after painting or a few weeks.
If you suffer from this problem then I hope maybe the solution outlined here will help you as much as its helped me. Let me just add that this isn't a definite method, there really are many ways to try and tackle this problem. What I can tell you is that I've tried several methods now and this is the only one thats got rid of it 100%.
Just in case you were wondering what other methods I've tried in the past, here they are:
All these methods have varying degrees of success and when used in combination can nearly eliminate the problem. I've highlighted 'nearly' because I've still observed a slight joint creep back using the previous best method I found consisting of sealing with the polyester resin and ~15 coats of 2k primer. But whichever combination of the above I tried, there's one annoying fact that remains: The joints do reappear to some degree.
Here's the problem:
After experimenting, reading and asking around there's a general concencus of the causes of this joint creep back and what will affect how quickly and severely this will be. Here's the most important:
Type of finish - A highgloss shows every imperfection quite clearly, matt finishes on the other hand are more forgiving. Another sure way to show joints is by using metallic finishes, the have a texture thats reactive with light. If the surface isn't uniform the reflective nature of the metallic paint will show this like a sore thumb.
Climate - The higher the moisture content in the air the more quickly MDF expands. A good example is leaving MDF outside as opposed to a stable and relative low humidity atmostphere such as indoors. Different parts of the world have different climates so this will vary between countries obviously but just be aware that temperature and moisture are big factors here.
Quality of your joints - flush and tight fitting joints obviously fare better than a rough joint or one that hasn't had enough pressure applied whilst gluing. If you can get away without using any filler in your joints then you know your doing well here.
Types of joint you use - This is the most important part. If you've got the tools, knowhow and time then avoid butt joints, these are the worse offenders. The best would be mitred joints as you then have the joint right on the angle transistion making it extremely hard to notice even without any preping.
Here's a diagram of a couple of joint methods:
When these types of joints are called for in construction, always go for at least the butt joint with MDF skin as this really helps to eliminate the issue and you certain won't need to do what I outline below.
The solution when you cannot avoid joints - veneer them
Sometimes no matter how good your intentions you simply can't avoid joints. One such situation is with laminations:
Without proper care, that could turn into a real mess once painted. You can of course try one or more of the methods I mentioned above such as sealing etc. But as I've already said, these are only partly effective. I recently struck upon an idea so simple and relatively easy that I'm amazed I hadn't tried it earlier.
Veneering cabinets is nothing new but veneering to avoid joints prior to spraying isn't something I've seen done before. Its actually pretty easy to do and I've taken a few photo's of the process which should be pretty easy to follow. For this example I show how to deal with a baffle made up from laminated layers of MDF.
Cutting the veneer:
This is the baffle prior to veneering. You can see all the joints very clearly.
_______________________________________________________
What we want to do is veneer just those sections with the joints. First step is to cut a piece of veneer roughly to size:
_______________________________________________________
Then place the face that you want to veneer on the veneer and carefully draw around with a sharp pencil:
_______________________________________________________
After thats done then cut out the shape. Be sure to allow a little excess just in case you weren't accurate in your marking - you can always take some off but you can't put it back. The veneer I used was a plain Beech and was suited to cutting with scissors. If your having problems with the scissor try masking tape on the veneer and cut through that which does help a lot. Otherwise a straight edge and a very sharp scalpal/craft knife will do the job.
_______________________________________________________
After that's done it should fit pretty nicely. Remember that a bit of overhang (about 1mm) is fine.
All bits cut.
I'll continue below.
There's a real problem spraying or painting MDF - it expands over time as it soaks up the moisture from the paint and the surrounding air. The net effect, as I'm sure your aware if you've tried this, is that joints reappear after a period of time. This could be a few hours after painting or a few weeks.
If you suffer from this problem then I hope maybe the solution outlined here will help you as much as its helped me. Let me just add that this isn't a definite method, there really are many ways to try and tackle this problem. What I can tell you is that I've tried several methods now and this is the only one thats got rid of it 100%.
Just in case you were wondering what other methods I've tried in the past, here they are:
- Level filling with Automotive and specialist MDF filler
- Sealing with upto a 5:1 PVA:water mix
- Sealing using Polyester Resin(Bonda Wood Hardener)
- Using 2k primer(primer with a hardner component)
- Extremely thick layer of Primer(around 15 coats)
- Various combinations of the above
All these methods have varying degrees of success and when used in combination can nearly eliminate the problem. I've highlighted 'nearly' because I've still observed a slight joint creep back using the previous best method I found consisting of sealing with the polyester resin and ~15 coats of 2k primer. But whichever combination of the above I tried, there's one annoying fact that remains: The joints do reappear to some degree.
Here's the problem:
An externally hosted image should be here but it was not working when we last tested it.
After experimenting, reading and asking around there's a general concencus of the causes of this joint creep back and what will affect how quickly and severely this will be. Here's the most important:
Type of finish - A highgloss shows every imperfection quite clearly, matt finishes on the other hand are more forgiving. Another sure way to show joints is by using metallic finishes, the have a texture thats reactive with light. If the surface isn't uniform the reflective nature of the metallic paint will show this like a sore thumb.
Climate - The higher the moisture content in the air the more quickly MDF expands. A good example is leaving MDF outside as opposed to a stable and relative low humidity atmostphere such as indoors. Different parts of the world have different climates so this will vary between countries obviously but just be aware that temperature and moisture are big factors here.
Quality of your joints - flush and tight fitting joints obviously fare better than a rough joint or one that hasn't had enough pressure applied whilst gluing. If you can get away without using any filler in your joints then you know your doing well here.
Types of joint you use - This is the most important part. If you've got the tools, knowhow and time then avoid butt joints, these are the worse offenders. The best would be mitred joints as you then have the joint right on the angle transistion making it extremely hard to notice even without any preping.
Here's a diagram of a couple of joint methods:
An externally hosted image should be here but it was not working when we last tested it.
When these types of joints are called for in construction, always go for at least the butt joint with MDF skin as this really helps to eliminate the issue and you certain won't need to do what I outline below.
The solution when you cannot avoid joints - veneer them
Sometimes no matter how good your intentions you simply can't avoid joints. One such situation is with laminations:
An externally hosted image should be here but it was not working when we last tested it.
Without proper care, that could turn into a real mess once painted. You can of course try one or more of the methods I mentioned above such as sealing etc. But as I've already said, these are only partly effective. I recently struck upon an idea so simple and relatively easy that I'm amazed I hadn't tried it earlier.
Veneering cabinets is nothing new but veneering to avoid joints prior to spraying isn't something I've seen done before. Its actually pretty easy to do and I've taken a few photo's of the process which should be pretty easy to follow. For this example I show how to deal with a baffle made up from laminated layers of MDF.
Cutting the veneer:
This is the baffle prior to veneering. You can see all the joints very clearly.
An externally hosted image should be here but it was not working when we last tested it.
_______________________________________________________
What we want to do is veneer just those sections with the joints. First step is to cut a piece of veneer roughly to size:
An externally hosted image should be here but it was not working when we last tested it.
_______________________________________________________
Then place the face that you want to veneer on the veneer and carefully draw around with a sharp pencil:
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
_______________________________________________________
After thats done then cut out the shape. Be sure to allow a little excess just in case you weren't accurate in your marking - you can always take some off but you can't put it back. The veneer I used was a plain Beech and was suited to cutting with scissors. If your having problems with the scissor try masking tape on the veneer and cut through that which does help a lot. Otherwise a straight edge and a very sharp scalpal/craft knife will do the job.
An externally hosted image should be here but it was not working when we last tested it.
_______________________________________________________
After that's done it should fit pretty nicely. Remember that a bit of overhang (about 1mm) is fine.
An externally hosted image should be here but it was not working when we last tested it.
All bits cut.
An externally hosted image should be here but it was not working when we last tested it.
I'll continue below.
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Joined 2002
Gluing the veneer:
I used contact adhesive to attach the veneer. Be careful which type you use, some react with cellulose paints. This one is fine though and I suspect because its a low solvent(low odor) type. If your using water based, acrylic, 2k, synthetic or any other paint then you should be OK with other brands/types. Always check the directions on the tin first though and if in doubt try a sample.
_______________________________________________________
Before sticking the veneer down go over the bits with a sander just to get everything back flat. Don't go crazy though otherwise the veneer you just cut to size won't fit.
_______________________________________________________
Apply a generous amount of adhesive to the open grain of the MDF, maybe two thin coats or one thick one however you want to look at it. Do the same with the veneer but don't apply quite as much. The key thing is make sure you've completely cover each surface, paying particular attention to the edges. Leave to dry for about 10 minute, you know its ready when you can touch the adhesive and find its not particularly sticky and almost dry.
_______________________________________________________
Now this is real important, very carefully line up the work before fixing into position. You've only really got one shot at it so go slow and be sure. Once that's done apply pressure by hand from the center out and then find something that you can use to apply a large amount of pressure to the veneer without damaging it. Use that to roll along the veneer and be sure to use whatever your pressing it down with at a slight angle on the edges to ensure they're thoroughly stuck down. Repeat that a few time until your happy.
_______________________________________________________
Repeat for all other parts.
That's it for now, I've still to trim the veneer so I'll take shots of the and upload them tomorrow.
I used contact adhesive to attach the veneer. Be careful which type you use, some react with cellulose paints. This one is fine though and I suspect because its a low solvent(low odor) type. If your using water based, acrylic, 2k, synthetic or any other paint then you should be OK with other brands/types. Always check the directions on the tin first though and if in doubt try a sample.
An externally hosted image should be here but it was not working when we last tested it.
_______________________________________________________
Before sticking the veneer down go over the bits with a sander just to get everything back flat. Don't go crazy though otherwise the veneer you just cut to size won't fit.
An externally hosted image should be here but it was not working when we last tested it.
_______________________________________________________
Apply a generous amount of adhesive to the open grain of the MDF, maybe two thin coats or one thick one however you want to look at it. Do the same with the veneer but don't apply quite as much. The key thing is make sure you've completely cover each surface, paying particular attention to the edges. Leave to dry for about 10 minute, you know its ready when you can touch the adhesive and find its not particularly sticky and almost dry.
An externally hosted image should be here but it was not working when we last tested it.
_______________________________________________________
Now this is real important, very carefully line up the work before fixing into position. You've only really got one shot at it so go slow and be sure. Once that's done apply pressure by hand from the center out and then find something that you can use to apply a large amount of pressure to the veneer without damaging it. Use that to roll along the veneer and be sure to use whatever your pressing it down with at a slight angle on the edges to ensure they're thoroughly stuck down. Repeat that a few time until your happy.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
_______________________________________________________
Repeat for all other parts.
An externally hosted image should be here but it was not working when we last tested it.
That's it for now, I've still to trim the veneer so I'll take shots of the and upload them tomorrow.
Re: Re: A 'how to' for solving MDF Expansion
Thinking about it a little more, the veneering example I've just done only treats all the end grain with no face involved ie. its all in the same plane of expansion/it expands at fairly similar rate. The veneering is perhaps only useful to lamination problem as shown in the example. It sounds like you should be changing your butt joints to one of those I suggested above, that solves the whole problem in one fell swoop.
Also consider that I'm assuming your also painting and sanding. The MDF has had its chance to stabilise at this point and the veneer acts as buffer:
(BLUE = Primer, RED= Veneer)
The thickness and type of veneer could be an issue too, the one I use is just under a mm thick and a type of Beech.
5th element said:
Thinking about it a little more, the veneering example I've just done only treats all the end grain with no face involved ie. its all in the same plane of expansion/it expands at fairly similar rate. The veneering is perhaps only useful to lamination problem as shown in the example. It sounds like you should be changing your butt joints to one of those I suggested above, that solves the whole problem in one fell swoop.
Also consider that I'm assuming your also painting and sanding. The MDF has had its chance to stabilise at this point and the veneer acts as buffer:
(BLUE = Primer, RED= Veneer)
An externally hosted image should be here but it was not working when we last tested it.
The thickness and type of veneer could be an issue too, the one I use is just under a mm thick and a type of Beech.
Thanks for the great documentation, ShinOBIWAN. I'm looking fwd to see more.
jpg
PS: Two pictures don't show for me:
http://img.villagephotos.com/p/2005-7/1050288/veneer05.jpg
http://img.villagephotos.com/p/2005-7/1050288/veneer10.jpg
jpg
PS: Two pictures don't show for me:
http://img.villagephotos.com/p/2005-7/1050288/veneer05.jpg
http://img.villagephotos.com/p/2005-7/1050288/veneer10.jpg
showflash said:Thanks for going the trouble of posting that. I really need all the tips on veneering I can get!
Thanks. I hope others can outline what worked for them too. This is just something that I found suited to my needs and methods rather than being the right or easy way. Its also probably not suited to every situation as 5th element has suggested. I'd likely have to try this out on the types of joints he's describing to be sure. It does work excellently for laminations though.
A wonderful effort
...Much needed.
Shin, If I may add a bit to your post #3, regarding alignment of the veneer before sticking it down: a slipsheet is very useful in preventing attachment before you want it to occur. A piece of brown kraft paper between the mdf and veneer will allow the final positioning to be done. It can then be pulled (slid) out. Once a corner or edge becomes bonded the alignment is fixed. This differs from what you describe in that it is an application from one end to the other. It can be useful for large pieces.
Thanks for doing this.
...Much needed.
Shin, If I may add a bit to your post #3, regarding alignment of the veneer before sticking it down: a slipsheet is very useful in preventing attachment before you want it to occur. A piece of brown kraft paper between the mdf and veneer will allow the final positioning to be done. It can then be pulled (slid) out. Once a corner or edge becomes bonded the alignment is fixed. This differs from what you describe in that it is an application from one end to the other. It can be useful for large pieces.
Thanks for doing this.
ShinOBIWAN said:
Using any waterbased primer(sealer) will create swelling problems. Use of PVA glue + water for a sealer should not be used with MDF(or anything else IMO, there are proper primer/sealers that are formulated for the job).
Using a solvent based primer/sealer will block the absorpsion of moisture, using it inside the box also will be even more effective - stopping moisture absorpsion from the inside also.
Using waterbased glue for joinery will introduce a small amount of moisture to the endgrain of the butt joint, which will cause a small amount of swelling. Allowing the glued joint to dry in a dry, heated area for a couple of days will reduce the swelling. To avoid this problem altogether, use a non-waterbased glue, such as polyurethane glue. Avoid using products such as silicone, or construction adhesive, or contact cement for wood joinery, as they do not provide a strong enough bond to avoid joint movement.
Joint movement is one of the primary causes of the condition you describe. Using a liberal amount of glue ensures good coverage, and makes the strongest bond. Tight fitting, well glued joints will not move. Use clamps to squeeze the joint together. Screws, for butt joint construction of MDF, should be avoided, as they will split the endgrain(even when a pilot hole is drilled). A tight fitting joint, using the correct glue, doesn't need screws to hold it together.
Re: Re: A 'how to' for solving MDF Expansion
The only benefit I found with the PVA/water was its ability to congeal all the lose fibres of the MDF so that when you sanded back you had a smooth surface to spray onto. It saved on primer but you get that with other methods and that was the only benefit really.
BTW Any liquid you apply to MDF causes some expansion, so its not just the waterbased finishes.
I see you mentioned specialist products, all the ones I've tried are useless. For example this so called 'specialist' MDF primer:
http://www.international-paints.co.uk/details.php?productid=96&primerprep=1&surfaceid=48
Which was pitiful and overall I can't think of anything worse. Avoid that one if you can.
Many commercial speaker manufacturers use mitred joints and high performance 2k primers which should tell you that there isn't an easy all in one specialist sealer/primer for MDF because if there was, these guys would be jumping all over it for the cost and time savings.
If the primer your using doesn't carry a big health warning label then it probably isn't good enough.
Good tip, this was a must back when I built car audio subs.
The joint never really moves even with just PVA glue. Its moisture causing the MDF to expand that makes them show through. So using alternative adhesives won't help the problem unfortunately.
I've tried alsorts there's no easy answer when using MDF. The real trick is minimising the problem before even starting treatments or solutions and the best advice I can give is to not use butt joints and go with either Mitred or a skin of 9mm MDF.
MJL21193 said:
The only benefit I found with the PVA/water was its ability to congeal all the lose fibres of the MDF so that when you sanded back you had a smooth surface to spray onto. It saved on primer but you get that with other methods and that was the only benefit really.
BTW Any liquid you apply to MDF causes some expansion, so its not just the waterbased finishes.
I see you mentioned specialist products, all the ones I've tried are useless. For example this so called 'specialist' MDF primer:
http://www.international-paints.co.uk/details.php?productid=96&primerprep=1&surfaceid=48
Which was pitiful and overall I can't think of anything worse. Avoid that one if you can.
Many commercial speaker manufacturers use mitred joints and high performance 2k primers which should tell you that there isn't an easy all in one specialist sealer/primer for MDF because if there was, these guys would be jumping all over it for the cost and time savings.
If the primer your using doesn't carry a big health warning label then it probably isn't good enough.
Good tip, this was a must back when I built car audio subs.
The joint never really moves even with just PVA glue. Its moisture causing the MDF to expand that makes them show through. So using alternative adhesives won't help the problem unfortunately.
I've tried alsorts there's no easy answer when using MDF. The real trick is minimising the problem before even starting treatments or solutions and the best advice I can give is to not use butt joints and go with either Mitred or a skin of 9mm MDF.
Re: Re: Re: A 'how to' for solving MDF Expansion
The loose fibres you refer to will congeal with any primer. The fact is, the water in the "homebrew" makes these fibres even more plentiful. Water will "raise the grain" on solid wood and plywood, it does the same with MDF. Professional furniture finishers(old school, not modern hacks) use water(a wipe with a wet cloth) to raise the grain before a final fine sanding. The trick is to let it dry completely. A quality finish takes time. A way to save time is to use a product that does not raise the moisture content of the wood (or wood product) that your working with.
BTW, not all liquids are equal in thier swelling ability. Water is the swelling champion. If you don't believe this, try this experiment:
Take two identical pieces of MDF and soak one in water and the other in mineral spirits( paint thinner). Leave them for about a hour, then check the results. You will see that the MDF soaked in water is considerably thicker than the other. The one soaking in paint thinner will be saturated with the solvent, but will be no bigger.
I mentioned no special products, just the suitable primer for whatever paint you are using. Sometimes, just the final paint thinned 10% is good enough for primer. BTW, by primer I am talking about the first finishing coat after final sanding, and before final painting. The primer will seal the wood, no need for a seperate "sealer".
I use mitred joints with great results, and I use butt joints with great results. The commercial manufactures are looking for the fastest finish possible, with the the fewest problems. If you have the time, knowledge and most of all patience, and using no magical cures, your joints will remain invisible forever.
If the joint is properly glued (with the proper glue), it will not move. You say it's moisture causing the MDF to swell that creates the crack. I say it's the MDF losing excess moisture that's causing the problem. If the glue has water in it, it's adding to this problem. Therefore, it's a solid idea you use a non waterbased glue for MDF.
The MDF needs to be at equalibrium moisture content before finishing, therefore using anything that introduces more water, is counterproductive.
Put it in a dry, warm area (maybe in the room they will end up in) for a couple of days to get the moisture content down. Then use solvent based primer to seal the box inside and out. Allow this to dry for a couple of days (yeah, what a drag), then it's ready for final sanding and painting.
ShinOBIWAN said:
The loose fibres you refer to will congeal with any primer. The fact is, the water in the "homebrew" makes these fibres even more plentiful. Water will "raise the grain" on solid wood and plywood, it does the same with MDF. Professional furniture finishers(old school, not modern hacks) use water(a wipe with a wet cloth) to raise the grain before a final fine sanding. The trick is to let it dry completely. A quality finish takes time. A way to save time is to use a product that does not raise the moisture content of the wood (or wood product) that your working with.
BTW, not all liquids are equal in thier swelling ability. Water is the swelling champion. If you don't believe this, try this experiment:
Take two identical pieces of MDF and soak one in water and the other in mineral spirits( paint thinner). Leave them for about a hour, then check the results. You will see that the MDF soaked in water is considerably thicker than the other. The one soaking in paint thinner will be saturated with the solvent, but will be no bigger.
ShinOBIWAN said:
I see you mentioned specialist products, all the ones I've tried are useless.
Many commercial speaker manufacturers use mitred joints and high performance 2k primers which should tell you that there isn't an easy all in one specialist sealer/primer for MDF because if there was, these guys would be jumping all over it for the cost and time savings.
If the primer your using doesn't carry a big health warning label then it probably isn't good enough.
I mentioned no special products, just the suitable primer for whatever paint you are using. Sometimes, just the final paint thinned 10% is good enough for primer. BTW, by primer I am talking about the first finishing coat after final sanding, and before final painting. The primer will seal the wood, no need for a seperate "sealer".
I use mitred joints with great results, and I use butt joints with great results. The commercial manufactures are looking for the fastest finish possible, with the the fewest problems. If you have the time, knowledge and most of all patience, and using no magical cures, your joints will remain invisible forever.
ShinOBIWAN said:
If the joint is properly glued (with the proper glue), it will not move. You say it's moisture causing the MDF to swell that creates the crack. I say it's the MDF losing excess moisture that's causing the problem. If the glue has water in it, it's adding to this problem. Therefore, it's a solid idea you use a non waterbased glue for MDF.
The MDF needs to be at equalibrium moisture content before finishing, therefore using anything that introduces more water, is counterproductive.
Put it in a dry, warm area (maybe in the room they will end up in) for a couple of days to get the moisture content down. Then use solvent based primer to seal the box inside and out. Allow this to dry for a couple of days (yeah, what a drag), then it's ready for final sanding and painting.
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