Finishing MDF

[Mike Lush]

Hi,

I've been doing some smaller projects using my shopbot and getting better at the finish sides of things, but I am unsure how to finish mdf. It seems the cheaper the material, the more finish time there is!

I've made some interior signs (large poker cards, sports symbols, etc.) for some friends but I am unsure on how to finish the mdf. Should I spray a primer on them and then hand paint? Any thoughts on this or a step by step instruction from the experts here would be really appreciated.

Thanks,
Mike

[Joe Chritz]

Paint with your choice of methods. MDF is designed to be painted and will take it by any means very well.

If I was doing signs I would look at using an airless sprayer of some sort or maybe something like tinted target USL or similar. If you have a spray system that will spray paint there are some good ones out there that aren't your home center variety latex.

Sherwin williams has a good one that people seem to like for paint projects. It is the proclassic series I think. Hit a real paint store and they will point you in the right direction.

Joe

[Larry Fox]

I have finished it with both regular oil paint as well as a more elaborite schedule with General Finishes Milk Paint (not real milk paint) as a base with good results.

[Prashun Patel]

I've primed (oilbased=good, shellacbased=best) and then painted (latexbased=good, urethanebased=best).

You'll get the best results (IMHO) if you thin the primer (mineral spirits if oil, alcohol if shellac) and then sanding between coats with 320.

Unless yr set up to spray or are spraying a LOT of sheets, the best application (IMHO) is with a shortnap (1/4 or less) roller.

If it's an outdoor sign, I'd just use a good exterior grade latex. It'll be flexible enough to withstand heat/freeze w/o cracking, and since yr not gonna put heavy things on it, blocking isn't an issue. If you have the time, then I'd thin the latex with 10% water or ethanol and roll it. The solvent'll help the naps flow out.

[Jamie Buxton]

I've tried spraying waterborne pigmented lacquer on MDF, and it did not work well. The finish raised the "grain" much more than on lumber. It took many coats, and sanding between coats, to get an acceptable finish.

[Phil Phelps]

MDF edges are like a sponge. Never spray it raw. You'll waste all your time and lot's of money. Rarely do I ever suggest buying cheap paint, but in this case use a cheap white, flat, interior latex. Use a short hair brush and hand edge, or roll and brush out. Let it dry and sand with 150 grit paper. Same procedure again and then sand with 220. Now you can spray it, or brush, roll, with most anything. I use lacquer over the latex, oil, and urethane. You must seal the edge and nothing works better than massaging it in with a brush. It's boring and tedious but must be done for a good finish. I'm in the trade show business and my stuff must look like museum quality art. Want to paint the face? Use joint cement compound and a trowel. Trowel on the compound and scrape it clean. Let it dry to a haze and sand with 220, repeat. Depending on the finish, you could use high solids auto lacquer and build it and sand it several times and make a glass finish. A slab door, for example, you trowel it twice, undercoat and paint. The compound method saves hours or time. This is not an experiment, I have done this for over thirty years.

[Chris Padilla]

I've done something like Phil does. I sealed the edges with spackle and then painted away but doing the face with joint compound...nice idea!

[Phil Phelps]

I've primed (oilbased=good, shellacbased=best) and then painted (latexbased=good, urethanebased=best).

You'll get the best results (IMHO) if you thin the primer (mineral spirits if oil, alcohol if shellac) and then sanding between coats with 320.

Unless yr set up to spray or are spraying a LOT of sheets, the best application (IMHO) is with a shortnap (1/4 or less) roller.

If it's an outdoor sign, I'd just use a good exterior grade latex. It'll be flexible enough to withstand heat/freeze w/o cracking, and since yr not gonna put heavy things on it, blocking isn't an issue. If you have the time, then I'd thin the latex with 10% water or ethanol and roll it. The solvent'll help the naps flow out.

You can buy an exterior grade MDF, but if I'm making out door signs, I'd use MDO, not MDF. Probably not worth getting into here, but a sign supply sells many MDO products that are primed, coated, vinyl coated. Come to think about it, I would much rather us metal. Outdoors is brutal.

[Mike Lush]

I haven't been able to visit for a few days, so thanks for all the comments. I have some experimenting to do I think!

[Prashun Patel]

Yeah, rethinking, I wouldn't use MDF with ANY coat outside. I did a test last year where I left a piece of MDF outside for a year (untreated). It swelled to 2x its original width.

For interior:
As for sealing the edges, I've had great luck with Zinsser BIN primer. One coat on the edge, then sand with 320 and finish w top coat. Worked perfectly and was a lot easier than mucking with j-compound.