Grit Selection for Sanding MDF
Question (WOODWEB Member) :
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.
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.
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