There are a few different ways to deal with this in order to get smooth edges.
1. Use an oil based primer. (No water for the wood fibers to absorb, no raised bumps.) 2. Cover the edges with joint compound and sand smooth before you paint. (I don’t do this as I think it’s too many extra steps…but it’s an option.)
3. My preferred method: go ahead and prime the entire piece, edges and all with water based primer and let those fibers swell.
When the primer is dry, take a sanding sponge and knock it down. It doesn’t require much effort, just sand over the little bumps until you have a silky smooth edge.
If you used a LOT of elbow grease when sanding and re-expose more fibers so you get more bumps when you paint, it should take very little effort to knock it down before you do a second coat of paint, or touch up that area. I’ve never had that happen though. There is generally enough primer still coating the MDF after the sanding to protect the fibers from the moisture in the paint.
(My CURRENT preferred Primer for MDF is Sherwin Williams “All Purpose Water Based Primer”.)
*Important Note: If the edge of your MDF has any saw marks, make sure you sand those off before you paint because the paint will only accentuate the flaws.*
Tags: DIY, Painting Tips, Tutorial