MDF: Masking pulled paint & primer off

[RossMosh]

The last few weeks I've been working on a sign project for the shop. Interior signs to hang in the windows as advertisement. I decided to use 3/4" cabinet grade MDF to do double sided signs. Well yesterday I go to peel the paint mask off and the masking pulls the paint & primer off. If I very carefully pull the masking off, I can get an almost acceptable sign, but ultimately none of them are really acceptable.

Material: Uniboard MDF
Primer: BIN 123 Shellac
Paint: Nova 127 Blue & Gold Rustoleum Rattle Can
Masking: Gerber Yellow Ultra
Spray Gun: Critter

I followed the pretty standard procedure for making these signs but there are 4-5 places where I think could have been an issue.

1. I didn't rough up the surface of the MDF. I was under the impression that the MDF was paint ready so I left it alone but going back I'm thinking running 180 grit paper before priming would have been a very good idea.

2. Temperature. It's been cold in NJ. I applied the paint in a fairly cold room. Around mid-50's. Then I bring the painted signs into another room to dry which is mid-60's. At night the temperature drops to 55-60*. It's possible the paint didn't 100% cure.

3. Left the mask on too long. CNC/Signs is not our primary business so I try to squeeze time in when I can. The mask was probably on the sign for a week or so before being removed.

[bleeth]

You hit it on all counts.
1. A scuff probably would have given it more tooth. I would have likely used a different primer. Your shellac may not have soaked in. One trick is to have the can warmer so the material is less viscous due to cold.
2. It needs to be well cured-if multiple coats they all need to cure before next coat. We paint lots of mdf for cabinet projects so are usually using catalyzed finishes and still give them a good day between sanding and next coat. More thin coats and less heavy can be a big help, although more work.
3. Leaving mask on that long is not a good idea.Particularly anything with yellow oxide in it like "gold."

I can't speak for the individual products you used but in general a good day to two between paint coats is called for in virtually all circumstances when using acrylics and mask. Also some masks are more tenacious than others. You may want to talk to Gerber tech

[RossMosh]

What technique would you recommend for priming? Based on what I'm seeing, the primer adhering to the MDF is my biggest issue at the moment. I've used sanding sealer previously. It was just a few extra steps that I wasn't really interested in doing.

As for cure times, I was pretty diligent in giving everything time to cure. At least that was my opinion at the time. I let the primer dry 3+ hours before touching it and it's supposed to only need 1 hour. I also let it dry a full 24 hours before applying paint. The paint I left for 5+ hours before touching it and then it was only to flip over the panel to paint the other side. Each side was given ~24 hours to dry before painting again and 24 hours before applying the masking. I know I left the masking on too long and that's on me, but I felt the primer and paint was all cured before going to the next step.

[joe]

Ross,

Sorry about the failure.

From what you've posted the problem began with the substrate. As you know MDF can be very smooth and sealing it off with a fast drying primer is part of the problem. But not all because most paints need a good tooth on which to grab. Had you scuffed or sanded the MDF with 180p and primer with a standard primer you'd been home free.

Alcohol primers are excellent to hide stains and hold on to surfaces like fiberglass. Thinned down Blockout White leaves a near perfect surface for most paints. By thinning it with paint thinner, it soaks in. Shellac won't deeply penetrate since it surface dries.

Joe

[gene]

Can u post a photo of the sign? Maybe I or someone can give a rescue idea on how to fix it

[Mayo]

You didn't mention if or how you cleaned the MDF prior to applying the primer. That is almost as critical as all the other things already mentioned.

When I have had to coat out MDF I would always clean it first with alcohol if using alcohol based primer or with mineral spirits if using an oil based primer. I would allow any excess to evaporate before coating with primer.

I would let oil based primers or latex primers dry a full day (or longer if necessary in cool temps) lightly sand the primed surface and clean it before coating with background paint.

Some masking adhesives will obtain a stronger bond the longer you leave it on - I would remove it when your last coat of paint is almost dry but not totally dry.

[joe]

Ross,

Thanks for posting this very interesting problem. Everyone wins when guys show their failures and successes giving us a heads up.

I'm thinking Mayo is on the right trail. For me I'd give this material a little sanding prior to priming. What we're looking for is a physical adhesion and sanding raises the fibers for this to happen.

It's so easy to take a few scraps of MDF, sand, prime and paint for conformation.

Joe