Finishing MDF

How to achieve a smooth finish on MDF, where the sealed face has been removed. November 29, 2000

Any tips on finishing MDF? We are using S-Williams primer and lacquer. Edges and routed flutes, etc. are absorbing the lacquer. They look rough and sanding doesn't help. There is no reduction of primer and a 3:1 reduction of lacquer.

Forum Responses
Step one: get a different product to prime with. Use a conversion primer or a high solids precat for a primer, or better yet a higher solids urethane, then you can get some build and sand them smooth. Just be sure to use a compatible topcoat with your primer. Sounds like your S.W. store or rep is not very helpful?

Bob Niemeyer, forum technical advisor

There could be 2 different things working against you.

1) The grade of MDF. Some lesser refined or less dense MDF could be your problem. I would suggest a double refined or 60lb MDF--these tend to be more compact or pressed in their make-up.

2) A low solids primer can also cause this problem. Look for a primer solids content around 28% to 45% by volume, not weight. These can come in the form of nitrocellulose, vinyl, pre-cat, or post-catalyzed products.

Most MDF requires at least two coats with proper sanding in between. On raw MDF start with 220 grit and don`t skip grits until you reach 320 grit. Then apply your 1st coat of primer. Then sand with 280 or 320 (don`t worry about sand through--the second coat of primer will cover it up), apply second coat and lightly scuff sand with 220 to ensure a good bond with the top coat (no MDF should show through at this point). If you require a second top coat, scuff the first then apply.

M.L Campbell has a great little pamphlet on finishing MDF from start to finish. Handy resource.

Finishing MDF requires a little extra care starting with the proper board, sanding, and of course a primer higher then 50% solids at the gun. Try keeping the viscosity 23-24 seconds on a ford #4 cup.

Note: for better results prime board right after you sand it, don't let it sit for days picking up humidity.

We like to use xim sau primer on MDF and we typically will add some more solids to make a brushable putty coat if the detail in the MDF looks a bit rough. There's a lot to be said for a brush application.

Sherwin-Williams makes a primer surfacer for MDF. I have had real good luck with glue size on the routed/machined edges. Mix yellow glue 50-50 or less with water, brush it on, let it dry, sand off the rough stuff.

I work with MDF on a daily basis. It could be of a high grade or it can be cut out of off - all. One tip is that it doesn't really matter what grade it is--you just need to pre-seal it.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor L:
I've been burning off the fuzz with an acetylene torch to good effect, then sanding to a smooth edge and priming.