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Painting MDF

Posted: Mon Dec 14, 2020 6:44 am
by gkas
I just submitted a project for a Mermaid. I've also got a couple of The Mouse that I've done for friends. They were done with box store MDF. I have them quarter a sheet, then I work off that. I'd like to try HDU (I think that is what it's called), but it looks expensive, since I give all this stuff away.

I get a LOT of fuzzies after I pocket the MDF signs. I pocket them to 0.15" with a 1/4" and 1/8" bit. I've sanded then tried coating with:
* Thinned white glue.
* SealCoat Shellac full and thinned 50/50.
* Krylon Clear

The uncut surface looks OK. The fuzzies still look terrible.

What If I cut the pockets, add lots of thinned SealCoat, then re-cut the pockets the next day with the same or 0.1mm deeper? I use downcut mills to push down the cuts, and there cuts are at the full 0.15" in one pass.

Is there a difference between the box store MDF and the stuff from a good lumber yard? I get my regular lumber at a yard that deals mostly with commercial accounts.

Re: Painting MDF

Posted: Mon Dec 14, 2020 10:05 am
by Adrian
There is a big difference between different grades of MDF when it comes to the core. You really need what is called "deep rout" or cabinet MDF. The core is far denser as it's designed to be cut into.

For the edges of cheaper MDF I've always used cellulose sanding sealer. I apply a coat, sand, apply another coat and repeat until the surface is glass smooth. That may work with the pockets as well but I wouldn't actually cut any deeper until after the first couple of coats as the MDF should swell slightly with the application of the sealer until it stops absorbing so much later in the process.

Re: Painting MDF

Posted: Mon Dec 14, 2020 2:49 pm
by wilkigr
The shop where I work part time does a lot of MDF painted cabinets. I do picture frames in my home shop.

We use a lacquer-based primer - 2 coats, then tinted lacquer - generally post cat. The finish is baby-bottom smooth and durable. I have had success with a water-based primer and a water-based post cat tinted lacquer from Spain. Since my home shop is in my basement, I like the low VOC aspect of it.

Re: Painting MDF

Posted: Mon Dec 14, 2020 4:13 pm
by dealguy11
There is a huge difference between box store MDF and the "good stuff" that shops like mine use for MDF doors. Generally, one of the best MDF's for painting is Weyerhauser Glacier Green (used to be called Plum Creek). Ranger Board is good. Medex paints reasonably well. I've heard good things about Medite but haven't tried it. If you can find a commercial shop to get it for you the wholesale price is of the nicer MDF is about the same as they charge for the icky stuff from the box stores.

Having said that, MDF is still hard to paint. We first sand it using a high-grit sandpaper - 220 or above. You're trying to "polish" it. Anything lower will open the pores and make it harder to paint. If you don't have to sand the factory finish then only scuff it lightly to get the paint to stick. Then you need to use a high-solids primer. We paint ours using fairly expensive materials that are not available to non-commercial users, specifically Sivam high-solids urethanes. The "high solids" bit is important. Check with your paint store. I have never had good luck with shellac. Glue might work. We apply 2 coats of primer, scuffing smooth with 320 after each coat. At that point it is smooth as can be and we put the top coat on.

Good luck.

Re: Painting MDF

Posted: Mon Dec 14, 2020 7:02 pm
by gkas
Thanks folks. I've copied this thread to my phone and I'll discuss this with my lumber yard on my next visit. I go to Reel Lumber in Anaheim, and they're really nice folks. I'm one of the few people not going out with a truck full of stuff. :D