Grit Selection for Sanding MDF

One school of thought holds that you can't sand MDF too fine, but that you can sand it too coarse. October 19, 2014

Question (WOODWEB Member) :
What grit do you stop sanding with on MDF? We use Festool sanders, which I like, but the MDF feels way to slick with 150#. I think 120# is better. The idea being that if it is too slick the primer will not achieve a good mechanical bond to the MDF. However, the rest of the team thinks that 150# is standard so that is what we should do. I am referring to the flats not the edges.

Forum Responses
(Finishing Forum)
From contributor R:
There are no hard and fast rules. If 150 is too slick, 120 is perfectly fine. Test the hypothesis on some samples. Decide yourself.

From contributor J:
Change your 150 paper more frequently. You need to keep a sharp tooth on MDF.

From contributor A:
You are sanding the faces of MDF before painting? We never sand the faces. The edges/profiles get 240 grit, spot primed and then 320 grit. Paint loves MDF. It is very porous.

From contributor R:
Thanks guys. All of this is pretty much what I thought too. These guys believe in those hard and fast rules. I tell them there is no such thing. Each job can be a little different than the last. I agree with not sanding the faces but sometimes we must because of flushing a joint, etc. I also agree with letting the primer get into those pores. That's the whole idea of a mechanical bond to the substrate.

From Contributor W:
There is no such thing as too slick on MDF. The primer or paint will have no issues penetrating the surface and creating a mechanical bond much deeper than mere surface scratches. I do 180 to level out the scratch pattern let by the wide belt sanders at the manufacturer. I wouldn't hesitate to follow that with 280. There is such a thing as too coarse a grit on MDF. The worst thing you can do is to rough up the surface and loosen the fibers before primer. You will end up putting massive amounts of material on the surface before it stops soaking it up. I've helped customers get great results using 50% less primer before paint. Much less primer sanded of to get to a very flat surface saves big bucks.