Sanding MDF

[pierhogunn]

okay, the last post on finishing MDF got a good answer, thanks Ray.

Ray mentions sanding the MDF to get a better coat, my question is, how far can I take the sanding, starting at 100 and going to 180 seems pretty reasonable, but what are the arguments for going to a higher grit, like 400 or 600?

I will only be putting latex paint on the thing with a 3/8" nap roller

[RayintheUK]

I was referring to the difference between the factory finish and a sanded finish of the faces of MDF. Many first-time users think that MDF is really smooth - which it is, compared to other sheet goods - and so they don't appreciate the amazing difference that a pass with 320 will make. After that, the original surface feels like sandpaper in comparison. Try it and you'll see what I mean.

I would never use 100 on the face - the lowest I'd start with would be 220 - I normally go straight to 320 on an area that is otherwise unblemished and using my Makita 6" RAS, one pass is enough.

If there are joints or glue-up areas that need sanding (to finish any rail and stile joints, for example), then I'll start by using the same grits as if the project was "real" wood.

Edges are a slightly more complex matter, but are worth taking trouble with. There, I'd start with 120 and go to at least 240. Sanding edges not only improves appearance, but gives a "glazed" finish that reduces absorbency for both glue and finishes.

BTW, I have tried 400 on the faces, but the difference was barely noticeable.

HTH

Ray.

[pierhogunn]

so just to reiterate,

you start with 220 and finish with 320,

you start the edges with 120 and then go to 320 so everything looks all nice and smooth?

awesome...

does MDF clog sand paper like finishes do?

[scorrpio]

Whatever grit you use, make sure to hook up dust collection with fine filtration (1 micron if feasible), and wear a respirator. I also recommend showering asap afterwards and laundering the work clothes before wearing them again. MDF dust is NASTY!!!!

[JimD]

I cannot imagine sanding anything with 320 or 400 that I was going to paint with latex paint. The paint is so thick it makes no difference past about 150 grit how fine you sand. To avoid MDF absorbing way too much finish on any cut edge, you should fill it first. Drywall compound works well and is cheap. There was a good article on painting MDF in FWW recently. You might want to see if your local library has back issues or perhaps you have a friend who subscribes.

I've made 10 or more raised panel doors with MDF panels that are painted. I did not sand the face at all and only sanded the routed edge a bit - no finer than 150 grit. I am confident they would look no different if sanded to 400 grit.

Jim

[Jeffrey Schronce]

I cannot imagine sanding anything with 320 or 400 that I was going to paint with latex paint. The paint is so thick it makes no difference past about 150 grit how fine you sand. To avoid MDF absorbing way too much finish on any cut edge, you should fill it first. Drywall compound works well and is cheap. There was a good article on painting MDF in FWW recently. You might want to see if your local library has back issues or perhaps you have a friend who subscribes.

I've made 10 or more raised panel doors with MDF panels that are painted. I did not sand the face at all and only sanded the routed edge a bit - no finer than 150 grit. I am confident they would look no different if sanded to 400 grit.

Jim

Second drywall compound on edges. You can get edges that look like glass. I use the techinque in FWW.