A 'how to' for solving MDF Expansion

[ShinOBIWAN]

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:

• 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:



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.


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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:



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Then place the face that you want to veneer on the veneer and carefully draw around with a sharp pencil:




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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.


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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.

[jleaman]

I'm totally closely watching this, i hope to learn a trick or 2

[ShinOBIWAN]

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.


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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.


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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.


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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.





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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.

[5th element]

The problem I have is joint creep UNDER the veneer... its exactly the same thing, but you see the end-grain-vs-not-end-grain reappearing over time on the finished surface.

[ShinOBIWAN]

Originally posted by 5th element
The problem I have is joint creep UNDER the veneer... its exactly the same thing, but you see the end-grain-vs-not-end-grain reappearing over time on the finished surface.

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]

Originally posted by ShinOBIWAN


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:

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.

Agreed, doing a mitred joint is just a pain in the **** imo so i'd rather not. Priming the MDF somewhat will probably help a lot to solve the issue.

[Cal Weldon]

I removed the excess posting.

[jpg]

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-...8/veneer05.jpg
http://img.villagephotos.com/p/2005-...8/veneer10.jpg

[Aengus]

jpg said:

PS: Two pictures don't show for me:

Me also.

Aengus

[showflash]

Thanks for going the trouble of posting that. I really need all the tips on veneering I can get! :-)