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Priming and Prepping MDF for Waterborne Top Coats
Question
This is what I'm doing:
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
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. Would you like to add information to this article? Interested in writing or submitting an article? Have a question about this article? Have you reviewed the related Knowledge Base areas below? 
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