Best methods for finishing MDF
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Re: Best methods for finishing MDF
I've used marine epoxy extensively in wooden boat building. (west systems or system 3 brands). While marine epoxy is much more expensive than polyester fiberglass boat building resin (6-10x) it is a much better product for wood. Fiberglass resin is porous and won't hardly stick to itself much less wood. That's why the fiberglass boats made in the early years had delaminated transoms and stringers, and osmotic blistering in the gelcoat.
Bondo is polyester resin. Much more porous than epoxy and can show seams by pulling moisture out of the wood itself.
The process for making joints in boat building is to:
1) seal coat the wood with clear epoxy,
2) filet the joint with a coat of epoxy mixed with sawdust (like peanut butter),
3) laydown fiberglass, kevlar or carbon cloth in the joint,
4) floodcoat the weave with clear epoxy,
5) coat again with sawdust mix to completely fill the weave,
6) top with epoxy mixed with microbaloons or talc. This mixture is much easier to sand than the sawdust mix.
Painting:
1) prime with marine grade high build primer, sand
2) re-prime low spots, sand again
3) paint - in my case I used off-white Imron high gloss. It looked like the finish on a brand new lear jet.
The result after 8 years in the FL sun is no visible seams in the paint.
This process is for making structural joints. For mdf joints I would think you could go from a clear epoxy seal coat straight to a finish coat with epoxy/talc, then prime and paint.Comment
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Re: Best methods for finishing MDF
Just had to add this....
NEVER never never never never induce or subject MDF to water, or anything water based. Period. After years of experience, and 2 years worth of reading all the debates about it, water or water based products directly in contact with MDF will cause swelling, damage, and other hassles.
Ive got a pair of TriTrix's that I merely took to a local automotive paint shop where 7 coats of pro grade high content urethane primer were applied (and sanded inbetween) and it all worked fine, especially on the end pieces and seams 10 month later.
Those that always state they use the standard mixes of wood glue and water for sealing MDF are NEVER telling you the whole story months later. It would be interesting to see their enclosures up close in person under bright natural lighting months after they are painted and completed...
There is no debate about it- water and water based products and MDF is a absolute no-no.Comment
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Re: Best methods for finishing MDF
So wood glue is now out?Comment
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Re: Best methods for finishing MDF
So what happens if you mix wood glue with something like isopropyl alcohol. Does it ruin the glue or would it just evaporate out of the glue super fast after being spread? I guess I could just try it but maybe somebody here already has.Comment
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Re: Best methods for finishing MDF
I have stated many times that I like the wood glue technique despite the fact that the seams eventually emerge... the amount that they emerge is acceptable to me.
The whole point of this exercise was to see if there's a technique that works better... if such a technique does in fact emerge, I'll happily change my approach... provided it is cost effective, doesn't create tons of nasty fumes, etc.
My Tritrix's are now more than a year old. The seams emerged a tiny bit, and don't seem to be getting worse. No regrets.Comment
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Re: Best methods for finishing MDF
It's the glue line. It restrains movement of the mdf at the seam.
Hopefully (and it looks good so far) the trench will be wide enough to bridge the effected area and help to feather or spread the visibility of the glue line.
The trench and fill is interesting...
But I would think that you would then simply have two emerging seams, one on either side of the trench.
Two different materials, thus two different behaviors in terms of expansion/contraction... don't have highs hopes for this one... hope I'm wrong...:o~99%
Make me an angel that flies from Montgomery
Make me a poster of an old rodeo
Just give me one thing that I can hold on to
To believe in this livin' is just a hard way to go
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Re: Best methods for finishing MDF
Incompatible chemistry?
You coated with lacquer primer then oil enamel, correct?
Usually you need to let things dry a considerable time before trying something like that! And it's still a crapshoot.
Lacquer primer and topcoat is no problem over epoxy. On glue.. it can cause cracking like that. Lacquer thinner is hot.
Enamel over lacquer, poor adhesion. The solvents in spray enamel are just hot enough to cause problems with some lacquers. Coupled with the slower evaporation of the less volatile solvents, they can trap the hot stuff for too long and cause problems with both lacquer and glue.
I like to stick with one chemistry so as not to introduce too many variables.
~99%
Make me an angel that flies from Montgomery
Make me a poster of an old rodeo
Just give me one thing that I can hold on to
To believe in this livin' is just a hard way to go
Comment
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Re: Best methods for finishing MDF
Incompatible chemistry?
You coated with lacquer primer then oil enamel, correct?
Usually you need to let things dry a considerable time before trying something like that! And it's still a crapshoot.
Lacquer primer and topcoat is no problem over epoxy. On glue.. it can cause cracking like that. Lacquer thinner is hot.
Enamel over lacquer, poor adhesion. The solvents in spray enamel are just hot enough to cause problems with some lacquers. Coupled with the slower evaporation of the less volatile solvents, they can trap the hot stuff for too long and cause problems with both lacquer and glue.
I like to stick with one chemistry so as not to introduce too many variables.
Last night I flipped the samples over and put a puddle of water on each one and let it sit for a couple of hours to see how the water penetrated each one. The Titebond II got soft to the point you could take a chunk out with you finger nail. The Bin 123 didn't get soft but it could be scratched off easy with your finger nail. It had no effect with the sanding sealer or the fiberglass resin. The water did not penetrate any of the samples to the point the MDF swelled.
So far there has been no changes in the samples for the last few days. All the samples with the trenches are holding with no sign of a seam. I am going to move them to my shower ledge today. Two of them are not sealed on the back or end grain and one of them is coated on all sides with fiberglass resin.
On some new samples I am going to try sealing with thinned down products with the trench method. In hopes thinned down will penetrate way down into the MDF and stablize it. After a couple of coats or until it doesn't soak in any longer I will let it dry good. Then fill the seam with polyester fillers. I am also going to try a shallower seam. I will see if I can take some 40 grit paper on a dowel and hand sand a groove. Then coat with like primer and paint.
DaveComment
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Re: Best methods for finishing MDF
DaveComment
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Re: Best methods for finishing MDF
I made some tower mounted speakers for a friends ski boat. but*-jointed 1/2" mdf. I coated the interior and exterior with two coats of marine epoxy, primed with high-build marine primer, finished with white rustoleum paint.
After 2-3 years outdoors in the the FL sun and rain, no seams showing. The white paint is a little chalky.
Marine epoxy is good stuff.
No seams, and this wooden boat is in the water 24/7.
Comment
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Re: Best methods for finishing MDF
I made some tower mounted speakers for a friends ski boat. but*-jointed 1/2" mdf. I coated the interior and exterior with two coats of marine epoxy, primed with high-build marine primer, finished with white rustoleum paint.
After 2-3 years outdoors in the the FL sun and rain, no seams showing. The white paint is a little chalky.
Marine epoxy is good stuff.
No seams, and this wooden boat is in the water 24/7.
http://www.rybovichandsons.com/last-...ll-125a/01.jpg
The thing I am trying to do here is to come up with an inexpensive process that is easy for someone with not so good of skills to do. That could really be done in the basement if need be. If it is as simple as a slight trench with some mud in it to hid the seam that would be great. I am also starting to think that most of the moisture problems with the MDF isn't from long term exposure as much as it is from the moisture from the finishing process.
DaveComment
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Re: Best methods for finishing MDF
There is a Composite Panel Association that might be able to help on this issue. http://www.pbmdf.com They even has a toll free number.
I had assumed that MDF was a monolithic uniform material throughout. But, in one of there publications "MDF Start to Finish" it states "Faces of MDF are generally more dense than the core,......." page 13 in:
This may be some of the problem since cracking is almost always caused by differing rates (or magnitude) of expansion and contraction. What does this mean? I have no idea. I posted so someone smart can find an answer (or tell me I'm full of it).Comment
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