It has everything to do with mdf.
Solid wood is much less problematic in this regard. And proper construction and finishing techniques used in solid wood construction do allow for movement and also as you suggest can be tailored to hide the effects of wood movement.
We aren't dealing with any of that here.
Further, solid wood is seldom painted in the manner that most speaker DIY'ers like. So we seldom have to deal with this problem in solid wood construction.
MDF is a super sponge. It takes on moisture and reacts to the moisture much more quickly than any solid wood.
True, mdf might not move as much as most solid wood species. But it will move at a much faster rate and under less extremes in rh than solid wood.
There are effective finishes. Those that contain water are not among them. Those that have a high permeability also are not suitable choices. Covering the joints is marginal. It can work. It is an in-between fix that will serve to disguise the movement in a specific manner. Veneering won't prevent movement any more than Elmers glue will.
In some cases, yes, there is no solution. The joint might show to some degree no matter what we do. But much can be done to mitigate the problem and even eliminate it in many cases.
Solid wood is much less problematic in this regard. And proper construction and finishing techniques used in solid wood construction do allow for movement and also as you suggest can be tailored to hide the effects of wood movement.
We aren't dealing with any of that here.
Further, solid wood is seldom painted in the manner that most speaker DIY'ers like. So we seldom have to deal with this problem in solid wood construction.
MDF is a super sponge. It takes on moisture and reacts to the moisture much more quickly than any solid wood.
True, mdf might not move as much as most solid wood species. But it will move at a much faster rate and under less extremes in rh than solid wood.
There are effective finishes. Those that contain water are not among them. Those that have a high permeability also are not suitable choices. Covering the joints is marginal. It can work. It is an in-between fix that will serve to disguise the movement in a specific manner. Veneering won't prevent movement any more than Elmers glue will.
In some cases, yes, there is no solution. The joint might show to some degree no matter what we do. But much can be done to mitigate the problem and even eliminate it in many cases.
ppfred said:
If the design allows, then mitered corners are the best solution and will prevent most visible signs of movement.
If there is a moisture problem you can still experience a hairline crack in the paint film along the mitered edge. Again, proper sealing will go a long way toward minimizing this.
A good solvent based sealer is also a good option but don't rely on it completely. You must make sure your substrate is dry.
Some sealants are more effective than others. Some are relatively ineffective even though they are solvent based. Polyesters in general are very poor. Anything with a high pigment content can be suspect. Anything easily sandable also can be a poor choice. The 3 items mentioned previously are very good, from 80 - 98% effective when properly applied.
Remember also, an enormous amount of moisture is introduced into the edges of mdf when you apply water borne adhesives. Some significant localized swelling will result. This can take days or weeks to work it's way back to equilibrium. So be patient, particularly in cool/humid conditions.
If there is a moisture problem you can still experience a hairline crack in the paint film along the mitered edge. Again, proper sealing will go a long way toward minimizing this.
A good solvent based sealer is also a good option but don't rely on it completely. You must make sure your substrate is dry.
Some sealants are more effective than others. Some are relatively ineffective even though they are solvent based. Polyesters in general are very poor. Anything with a high pigment content can be suspect. Anything easily sandable also can be a poor choice. The 3 items mentioned previously are very good, from 80 - 98% effective when properly applied.
Remember also, an enormous amount of moisture is introduced into the edges of mdf when you apply water borne adhesives. Some significant localized swelling will result. This can take days or weeks to work it's way back to equilibrium. So be patient, particularly in cool/humid conditions.
lemans23 said:
You can't have those joints showing they ALWAYS come back even if you go absolutely insane with the primer. Give it a few months and you'll be seeing them again.
There's a couple of methods I use:
Veneer over the top then spray that. This works brilliantly, be sure to apply a good amount of primer and sand smooth though. Choice of veneer is important, go with a very plain birch and ensure the leaves are flat, this will help with less work flating the primer.
Add a 9mm MDF layer over the joints and bevel the edges with router so the the transition to the bevel from the cabinet wall has the joint slap bang on the edge - really hides it well.
If can't do the above, then the best you can do is the make the most of bad situation. This means something like a polyester or resin based sealer(don't use PVA its useless for this purpose), apply lots of the then sand smooth afterward then go to primer and apply 10+ good thick coats and then sand flat and apply finishing coats. Even with all this work you'll still have a faint impression of them given enough time.
Seriously those are the only ways that I know of to either stop or ease the problem.
There's a couple of methods I use:
Veneer over the top then spray that. This works brilliantly, be sure to apply a good amount of primer and sand smooth though. Choice of veneer is important, go with a very plain birch and ensure the leaves are flat, this will help with less work flating the primer.
Add a 9mm MDF layer over the joints and bevel the edges with router so the the transition to the bevel from the cabinet wall has the joint slap bang on the edge - really hides it well.
If can't do the above, then the best you can do is the make the most of bad situation. This means something like a polyester or resin based sealer(don't use PVA its useless for this purpose), apply lots of the then sand smooth afterward then go to primer and apply 10+ good thick coats and then sand flat and apply finishing coats. Even with all this work you'll still have a faint impression of them given enough time.
Seriously those are the only ways that I know of to either stop or ease the problem.
as a carpenter..
the solution we use for MDF joints is.. of all things.. drywall joint compound! you put the mud on the endgrain (of sorts.. since it's mdf.. paperboard tree..), and then sand it. the gypsum seals the fluffy fibers, and acts as a 'grain sealer'.
it's surely cheap enough! give it a try!
d
the solution we use for MDF joints is.. of all things.. drywall joint compound! you put the mud on the endgrain (of sorts.. since it's mdf.. paperboard tree..), and then sand it. the gypsum seals the fluffy fibers, and acts as a 'grain sealer'.
it's surely cheap enough! give it a try!
d
Different type of movement and different set of problems in this case I think.
The original post was about the uneveness of the levels where sides join baffle if my understanding is correct.
Not cracks in the paint film on the joint line.
Not ends or edges soaking up finish or getting fuzzy or rough.
Not the difference of rate of expansion of width to length that is common in solids and plywood.
If my understanding is correct: the problem is that the mdf is shrinking and/or expanding in thickness. Something mdf does to a much greater degree than most any other product suitable for loudspeaker enclosure construction.
This can be lessened or eliminated if...
The original post was about the uneveness of the levels where sides join baffle if my understanding is correct.
Not cracks in the paint film on the joint line.
Not ends or edges soaking up finish or getting fuzzy or rough.
Not the difference of rate of expansion of width to length that is common in solids and plywood.
If my understanding is correct: the problem is that the mdf is shrinking and/or expanding in thickness. Something mdf does to a much greater degree than most any other product suitable for loudspeaker enclosure construction.
This can be lessened or eliminated if...
ppfred said:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.