Priming and Prepping MDF for Waterborne Top Coats

Advice on prepping MDF to achieve a smooth finished surface. February 2, 2011

I've been having difficulty achieving a smooth primed surface on MDF using Becker Acroma Akvasurf.

This is what I'm doing:
- sanding MDF to 400 with orbital
- first coat of primer 3-4 mils wet
- scuff with 400 by hand lightly (lots of raised fibres)
- second coat of primer 3-4 mils wet
- scuff with 400 by hand lightly (still a few raised fibres)

Does it need a third coat? Seems like a lot of material to put on. I'm trying hard not to sand through the raised fibres after the second coat but it seems inevitable. I'm following M L Campbell's guidelines for spraying MDF with waterborne. Does anyone have a different method?

Forum Responses
(Finishing Forum)
From contributor W:
Yes, the MDF will absorb a lot of finishing material - much more than wood or veneer. You may need to add a coating layer. I prefer to use polyurethane coating for the first layer. The polyurethane will give better build since it is catalyst clear coating.

From contributor M:
Are you getting raised fibers on the flat surfaces, or is this mostly on the edges where you have cut or routed? We prime MDF with a fast dry oil based primer from Sherwin Williams, usually 1 coat, sometimes 2, followed by oil or latex enamel paint. We sand in between, but we never get any raised fibers except on the edges. Before priming the first coat, we dress the edges by applying a sizing and then sanding, sometimes once but usually twice.

From the original questioner:
My edges are turning out perfect (sanding to 600 before I shoot), but the surface is the problem. I'm starting to have a bit of success with tacking up 2 mils for 5 minutes then hitting it with another 2 mils, sand and repeat. I'm trying to stick to waterbased products because I'm not well enough equipped for solvents.

From contributor C:
The roughness on the faces of your MDF could simply be sanding dust. Try wiping the surface with your bare hand before spraying your first coat. Keep wiping until no dust is on your palm. If the surface has dust on it, the water base will swell it up.

From contributor R:
How much coating depends greatly on the quality of MDF. If you are using ML Campbell topcoat, you should use Agualente primer on MDF. You will still need 2-3 coats of primer depending on quality of MDF.

From contributor J:
I have been using General Finish Enduro undercoat and pigmented acrylic topcoat and it works well even on low grade MDF. If I shoot on a nice thick layer of undercoat with my airless, I can get away with just the one coat and then do two coats of color. I don't see how you could need to pre-sand your wood to 4 and 600. Everything in my shop stops at 180 or 220 before finishing and I only sand my MDF where there are edges, or if it seems to have a sheen or glaze that the undercoat will really bite.