MDF Help

[Tom Pendo]

My daughter asked for a desk to do her homework. I had an extra peice of MDF laying around so I used PVA to seal it. I then sanded it to a very smooth finish. I was quite impressed with the results.

I gave the desktop a day to dry completely and started with the painting. I choose an oil based enamel from Home Depot and sparyed it for the first coat. I then used a ROS to sand it smooth. I have spayed the second coat and the top look great, but I am able to take my finger nail and make and impression in the pait. It's been over a week since I painted the desktop and I'm suprised that I can make an impression with my finger nail.

What did I do wrong???

The PVA method was quick and easy. Was there a better choice of paint?

Any help would be appreciated.

-Tom

[Jim Becker]

I'm curious as to where on the MDF you used the PVA? (Glue I presume) Just the cut edges or the whole surface? I usually use fast dry spackle for MDF edges, usually two applications with sanding after each to smooth. Then a primer compatible with the intended top-coat.

An oil-based enamel like you used or a 100% acrylic water borne product (NOT latex paint) should normally be great choices for this kind of product. But remember cure time for a full cure can be a very long time for some products. If you're just finding the surface to take an impression, but it's not coming off, you probably just need more time for a full cure.

[Jason Beam]

I think the PVA in question might be PVA Primer - like for bare drywall. It doesn't soak up quite as bad as regular primer and it seals real well.

I'm with Jim, it sounds like you just need more cure time. The walls in my shop are 100% water based acrylic and they took a few weeks to toughen up.

[Tom Pendo]

The PVA was actually white glue. It went on really well and sanded well. I did put it on all of the MDF not just the ends. Can an oil based paint take over a week to dry? I'm guessing it should setup and be hard as nails, right?

Is there a better method for painting MDF? I absolutely want a smooth finish without brush marks. I've tried rolling and have gotten horrible results, that's why I went the spray route. If this desktop works out I want to replace my glass top on my dinning room table with a similar top but sprayed MDF. So far, I'm not impressed with the results.

Again, thanks for the help.

[Joe Chritz]

It could take a long time to cure.

If you can spray HVLP you can try using a colored finish like Target USL tinted with universal color. I think you can buy them pre colored from homestead finishing.


Joe

[Prashun Patel]

I've used MDF on shelves before and found Zinsser BIN primer or shellac to work well on edge sealing. Spackle (all respect to JB) MIGHT crack over time on a desk.

I also found oil paint or even a urethane paint to hold up well without 'blocking' or softness.

While it's more work, you can also paint with latex paint and then coat with a waterbased polyurethane. That worked well for me in my daughter's closet.

[Jim Becker]

Shawn, the spackle is only used to fill the rough cut edges and as such, isn't a continuous coating, so cracking shouldn't be an issue. It's sanded back several times until a suitable smooth surface on those cut edges is achieved. Some folks use Bondo, but I never felt the need.

Tom, the uncut surfaces of MDF don't benefit from the PVA glue, IMHO. There is already a primer ready surface for painting.

Yes, paint can take a long time to fully cure a month would not be unusual. There is a big difference between "dry" and "cured". But as paint, it's not necessarily going to be "hard as nails", either. That depends upon the specific product. Commericially available products that are "painted" typically are finished with lacquers, conversion finishes, etc., rather than paints available in the retail environment. These are specifically designed to cure "hard", yet be thin and smooth. Like Joe mentions, I use Target USL (a water borne acrylic product with some of the properties of lacquer) that is tinted...my whole kitchen was done this way and still looks nearly new in 2008 even though the work was done in 2003.

[Tom Pendo]

This is wonderful help.

Since this desktop can be a beta test (I'm an IT guy), would you recommend that I redo a desktop with the Target USL product? I want her to be able to write on the surface. And I'd like to use the same methodology for a replacement dinning room table top.

Additional question... would a polyurethane help with the hardness or wearing over time? Can that be used over paint?

[Phil Phelps]

Tom, curing can take a month or more. I don't know if you'll find anything as hard as nails other than an epoxy paint. Bin pigmented shellac is a great sealer, undercoat, but curing take time, also. Why not use laminate? Many colors to choose from and you won't be marring it anytime soon with pen or pencil.
Just a thought.