Skim Coating MDF for Paint

[Brett Bobo]

Is it advisable to skim coat MDF with drywall mud prior to priming and painting given the porosity of MDF? I recall seeing this tip somewhere but can't seem to locate it now. My thought was that skim coating the surface will fill the pores and reduce the ability for the grain to raise and become "hairy." Note: the paint will be a SW oil based enamel. Any advise would be appreciated.

Thanks in advance,
Brett

[Prashun Patel]

I'm not a pro, but I have done a few closets in painted mdf.

An oil based enamel won't raise the grain of mdf, which will only swell if exposed to a waterbased paint.

Drywall mud will make the surface brittle, and is just a mess. Sometimes people use it on the edges, which don't tend to get sanded as well as the tops which keeps them really porous.

I've found a better solution is to use shellac or a shellac-based primer like Zinsser BIN. Either will seal the MDF, will dry quickly, and will sand super smooth with a quick swipe of 220 or 320 grit sandpaper.

Then you can apply your enamel over top. I recommend a low nap or foam roller for the flat surfaces. That's the quickest route to a glassy surface (unless you will be spraying).

[Brett Bobo]

Thanks, Prashun. Actually, I forgot to ask about the use of the Zinsser BIN primer instead of the mud skimcoat as I've seen this recommended before as well. How many coats of primer, typically? Also, do you only sand the profiled portions of the MDF or all of the MDF?

I'm considering spraying this but have seriously considered rolling it as most of the millwork has simple profiles, some of which are just S4S stock.

[Steven Hsieh]

Thanks, Prashun. Actually, I forgot to ask about the use of the Zinsser BIN primer instead of the mud skimcoat as I've seen this recommended before as well. How many coats of primer, typically? Also, do you only sand the profiled portions of the MDF or all of the MDF?

I'm considering spraying this but have seriously considered rolling it as most of the millwork has simple profiles, some of which are just S4S stock.

The problem with mud, that if don't remove the dust, any paint will not adhere to the surface. Water Also.
There is a no need for it. Any top coat will seal it.

If you apply with water base products, it will raise the grains. That is why you should use oil base.

To seal the grain use quality oil primer, Zin Bin primer and Cover stain Primer both work also. It won't fill in the grains, but seal it.

1 coat of primer.
You can lightly sand after primer has dried.
Top coat with 2 coats of enamel.

[Scott Holmes]

Drywall mud may cause swelling of the MDF...it has water in it.

One or two light coats of shellac is all that's needed.

[JohnT Fitzgerald]

I think I saw that same tip in ShopNotes a few years back. It's more important on the end "grain" than on the normal surface, but I tend to believe that the Shellac advice you see here might be a better approach. let us know what you choose and how well it works out.

[Phil Phelps]

Maybe you saw it from me, since I've been a fan of drywall mud on MDF for over 20 years. I guess one can make a mess of most anything but, if you have a large, flat surface, like a slab door, you can use a broad knife and trowel on regular dry wall mud. Trowel it off, don't leave a build up. When it dries to a haze, use a ROS and 220 grit. You're ready to prime. It doesn't get any more simple. I have MDF doors, cheap, open grain mahogany doors, all filled with drywall, primed and painted and they look super. Edges are a different story. Shellac, preferably Bin pigmented shellac, is a great sealer for MDF. So is cheap, white, interior flat latex . Look, I've painted thousands of MDF cut out letters. I have to hand edge every one of them. This is the most boring, monotonous job on the planet. I've tried every short cut I can think of. Cheap, white latex fills the pores faster than anything else. (That's hand brushing the edges) Takes two coats and sanding in between, but they look like plastic when I'm done. Do as you wish, I've been doing this for a living for 44 years.

[Joe Chritz]

BIN here. I spray the profiles only, scuff sand then spray the entire panel.

Hand brushing any finish is going to fill faster and one coat may be enough. I never saw much reason to fill the flat undisturbed areas before paint but I don't do a lot of MDF panels.

Joe

[Pat Barry]

Drywall compound is a terrible choice unless you have a lot of skill. Phil has the technique down pat after 40 years. You will be sanding and sanding and creating dust and a less level surface than you started with. Go with the BINs primer sealer. If you have edges then sand them smooth first. Not seeing a benefit to the drywall stuff here at all.

[Phil Phelps]

Drywall compound is a terrible choice unless you have a lot of skill. Phil has the technique down pat after 40 years. You will be sanding and sanding and creating dust and a less level surface than you started with. Go with the BINs primer sealer. If you have edges then sand them smooth first. Not seeing a benefit to the drywall stuff here at all.

There is nothing to it. You don't leave a build up. You are just filing the pores. Trowel on, trowel off. It couldn't be easier. Of course my ROS is hooked up to a Porter Cable vac, there is no dust. Put a fan on it, dries in minutes. It covers like two coats of paint. And a lot less messy than paint.