A question about mdf

So my good customer has asked me to make yet another style of sign for him.
I have never worked with MDF but thought I would give it a try in hopes of it not warping like other woods.
I made this new sign with the following process after machining.

  1. completely sealed all surfaces with sanding sealer.
  2. painted the black with enamel
  3. painted the green with enamel
  4. sprayed all surfaces with minwax polycyclic.
    So today the sign is warped.
    I know that MDF draws moisture, but I thought I could prevent that by sealing it up.
    Anybody have a suggestion as too what I did wrong?

What thickness did you use. By the way I have used 3/4 and never had a problem. I personally would spray the piece with a poly urethane spray. What type of sealer did you use.

Its 1/2 inch thick.
I first used sanding sealer, then after painting, I used minwax Polycrylic.
I like to use all material that are water clean up.

I did some MDF pumpkins for Halloween that were outdoors for a few days without any warping. I used spray paint.
They were smaller so that may have been a factor too.

I once asked on a different forum what people were using to seal their MDF signs against weather. The consensus was fiberglass resin (just the resin no fiberglass). But I have yet to try it out.

FYI: To save time when I do MDF signs I like to do them as a 2 part profile cut, instead of a pocket operation. The top layer profile cut out, the bottom player with a shallow profile cut so I have a alignment guide. Here is an example.

I started by putting sanding sealer on all surfaces and sprayed on the polycrylic on all surfaces after painting.

Maybe the humidity was higher than normal before sealing? I’m just tossing a thought out there, maybe it was in the material and then got trapped?

Looking at the picture he didn’t go to deep and weaken the structure. If he had gone ,125 and that leaves .375, That should be plenty strong.

Its actually .25 deep.
You may be on to something.

The next one I do will be 3/4 inch.
Robert, what sealer do you use.
I’ve seen you talk about sanding sealer a lot, and that is actually why I used it.

I would skip the sealer and start with rustoleum enamel. use a foam brush and foam roller for a super smooth finish. I make these to make cheap tubs look more expensive. Could not be in a more humid area and never had a problem. the enamel is like a force field.

1 Like

I created my Game of Thrones house sigils in 3/4" mdf, I coated in Minwax Wood Hardener, carved, applied hardener again, sanded lightly and then painted and applied polycrylic.

As a test, I submerged one (bad carve) in a bucket of water overnight and saw no deformation that indicated water damage…

That looks sharp and clean. I would like to do that in my dining room with raised panels.

That’s exactly what I’m going to do.
3/4" MDF .125" deep
Thanks for everyone’s input.


Did the work piece cup up from the back?

Well, this is. Minwax was mentioned.


I clean up my brush with warm water and soap

I went back and read Roberts earlier post.
He uses shellac.

I think you have identified your problem.

Ill try the next one with shellac.
also using 3/4 inch

I like the aerosol shellac. Easy Peasy.

My understanding of his project is that it was 1/2" mdf and the pocketing was .25" deep.

A water based product was then applied to the back. That is why it cupped.

A solvent based sealer on mdf will serve him better.