Best methods for finishing MDF

[davepellegrene]

Re: Best methods for finishing MDF

Dave,

Wood, wood products and almost every finish imagineable have very low thermal coefficients.
Typically in the range of less than .005% at a temperature range +90F to -20F.

Relative humidity is the thing to watch. Dimensional change is orders of magnitude greater for just a few percentage point change in RH.

If you want to see some paint films crack or craze, then freeze the samples.
Usually only artist type emulsions and antique finishes will crack or craze at these low temperatures. Below -20 it's another game.

Any thoughts on why the paint cracked on the Titebond and Epoxy samples. They have been sitting in 68 degrees for over two weeks.
Dave

[dwigle]

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.

[HuskerNation]

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.

[Erich H]

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.

There is no debate about it- water and water based products and MDF is a absolute no-no.

So wood glue is now out?

[evilskillit]

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.

[lunchmoney]

Re: Best methods for finishing MDF

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.

Oh really? NEVER? NEVER EVER?

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.

[Wolf]

Re: Best methods for finishing MDF

So wood glue is now out?

If you don't want to mix with water, just use straight Elmer's white-glue, and rub it in with the digits...
Later,
Wolf

[bobbarkto]

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

[bobbarkto]

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.

Any thoughts on why the paint cracked on the Titebond and Epoxy samples. They have been sitting in 68 degrees for over two weeks.
Dave

[davepellegrene]

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.

I sanded the sample down in one spot with 600 wet. The lacquer definitely cracked. Doesn't look like the glue cracked though.
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.
Dave

[dlneubec]

Re: Best methods for finishing MDF

Dave,

Have you thought about a narrower 45º channel routed right on the seam?

[davepellegrene]

Re: Best methods for finishing MDF

Dave,

Have you thought about a narrower 45º channel routed right on the seam?

It may work Dan. I can always give it a try.It would be less work. My only concern would be that almost all the expansion in MDF is in the thickness at the edge, so any sharp angles will tend expand mor to show a seam line. What my thinking is, which could be wrong. If I keep the sharp angle of the joint down from the surface and fill with something flexible, when the MDF expands, it will buffer the joint. Then at the surface of the trench,if it is a slight slope were it is feathered into the filler, it will be less noticeable. In the sample pic below the one on the left has the sharp angle at the surface. The sharp angle is were the expansion will show up the most. Without a buffer over it it will show a seam. On the sample to the right the sharp angle is lower and covered with a buffer material and should stay hidden. Then at the surface it has a low slope were it is feathered into the filler and shouldn't show the expansion. I also think the filler needs to be feathered past the joint a ways and feathered into the flat part of the MDF.


Dave

[dwigle]

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.

http://www.rybovichandsons.com/the-process-03.html#gall-process/34.jpg

No seams, and this wooden boat is in the water 24/7.

http://www.rybovichandsons.com/last-launched.html#gall-125a/01.jpg

[davepellegrene]

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.

http://www.rybovichandsons.com/the-process-03.html#gall-process/34.jpg

No seams, and this wooden boat is in the water 24/7.

http://www.rybovichandsons.com/last-...ll-125a/01.jpg

I don't doubt that a bit. The problems I have with that method is the expense. You may have had it laying around from boat building. It would be very expensive to buy to coat a pair of speakers. I recently redid a surface on a water ski jump. I used CPES then Epoxy high build paint. Then coated with boat enamel. I got the products from Jamestown Distributors. Stuff did a great job, but again really expensive. Not to mention I am sensitized to epoxy.
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.
Dave

[MrkCrwly]

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:

http://www.pbmdf.com/CPA30/files/ccLibraryFiles/Filename/000000000729/MDF%20Start%20to%20Finish.pdf

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).