Sanding MDF Before Finishing

A fine grit is best with MDF because of the material's fine, fuzzy fibers. February 27, 2015

Question (WOODWEB Member) :
Has anyone had problems sanding MDF before spraying a lacquer finish? Our finisher has had nothing but problems with getting the panels flat enough to spray. We have to sand each panel by hand with a hard block on an angle until the panel is completely flat. This is very time consuming. Does anyone have any ideas?

Forum Responses
(Architectural Woodworking Forum)
From Contributor S:
Use 320 grit or so - remember itís paper not wood.

From Contributor W:
Are talking raw MDF with the factory surface? That it's usually very flat.

From Contributor W:
I use two-three heavy coats of automotive primer/surfacer and then block sand with 320 before top coating.

From Contributor W:
To contributor W: I see a lot of shops using that technique. It usually means a lot of material built up and then a lot of it sanded off and sucked up the dust collector. When I have people sanding and priming routed or sanded MDF, I like to have them try a very light coat of primer followed by polishing with a 280 or 320 grit, just to remove the raised fibers and slick up the surface. Then the next coat doesn't need to be so thick because the surface doesn't pop as much. I find soaking with a lot of primer to be often counterproductive.

From the original questioner:
Have you ever sanded your MDF panels through a Heesemann sander before primer?

From Contributor W:
A Heesemann sander would be a great choice for sanding out sanding marks coming from the factory that produced the MDF. It might not be so great for calibrating it if that is what you need. Just remember to keep the grit pretty fine so you don't start pulling up fibers and creating a fuzzy surface that will need a lot more primer to seal. I would start with light pressure on a 220 grit silicon carbide and maybe consider a 280 grit.