Wayne Parham Messages: 18686 Registered: January 2001
Illuminati (33rd Degree)
I'm not a woodworker, I'm an electronics man. So this is just what I've gleaned from the folks that do the work. I think it's good advice:
1. Seal edges with putty. Drywall mud is fine for this. You just don't want edges exposed because they're more porous than faces, and tend to swell. After the mud dries, sand smooth.
2. Use a solvent-based primer to seal the surface. That's lacquer, oil or alcohol. Don't use any water-based primer. Even a slight amount of humidity brings fibers to the surface, defeating the purpose. Standing water swells MDF and turns it into mush. After the primer dries, sand smooth.
3. After the primer is applied, you can use a water-based latex finish coat if you want, but it's probably best to use a solvent based finish coat like enamel or lacquer. Water is just not a good thing for MDF.
NWCgrad Messages: 143 Registered: March 2010 Location: Frederick, United States ...
Master
I just painted the MDF front/rear baffles of my 4 Pi speakers and they turned out pretty decent (even survived a trip to SE Asia on a very SLOW boat).
For sealing the edges of the MDF I used Bondo, but the suggestions from Wayne are also reported to be effective. Many people use a 1:1 dilution of wood glue with water - there are many threads indicating this is great/stupid.
After sealing the edges and sanding smooth, I primed the area with two coats of Zinsser BIN shellac stain-killing primer-sealer. I had Home Depot tint the primer gray. The primer dried very smooth and only required light sanding before painting. For paint I went with Behr Premium Plus Exterior satin enamel (100% acrylic latex). I received a lot of feedback on multiple forums espousing the advantages of both 100% acrylic latex and oil-based paint. In the end I went with acrylic because I had to pack the speakers for shipping within a week of painting and the oil-based takes much longer to cure. I had no issues with MDF swelling from the paint, probably because of the two coats of sealer/primer. The exterior paint is really thick and the roller tended to slide rather than roll, but other than that I think it was effective.
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