Problems Painting MDF

[needlesmcgirk]

Alright, so back in January I painted my cab with some gloss latex based black paint with a very fine foam roller.  I sanded it down and primed it first before I painted, and it end up being very "glittery" looking, and not very smooth at all.  I chocked this up to the sudden cold snap and the paint not drying very well.

I sanded the whole thing down and decided I just need to start over when the weather is warmer.

I finally got back to this and started painting it again and pretty much the same thing happened.  Right now I am very discouraged and I'm not really sure what I can do at this point.  Perhaps try an oil based paint?  Perhaps sand the whole thing down and use rubbing compound and hope for the best?

Has anyone had a similar experience and may have some advice for me?  Any help would be greatly appreciated.

[knave]

Someone once reccomended making a thinned mixture of waterproof wood glue and coating the MDF with that to seal it.  then sand and paint.  I plan on trying it once I get my build started.

[ScottS]

Getting gloss paint to looks good is virtually impossible for most mortals. If you're not a professional painter, you might do better with semi-gloss or matte paint.

That said, there are a few things you might try:

1) If you're using latex paint and ending up with brush marks, try adding some Floetrol to the paint. Floetrol thins the paint, but doesn't ruin it like water will, and helps it to "flow out" more evenly before it dries. I've had good luck with Floetrol and smi-gloass paint. Haven't been brave enough to try gloss...

2) You might need to try wet-sanding the paint between coats.

As knave suggests, you'll definitely want to start with a very smooth surface before you try to apply the paint. If you primed but didn't sand, that may be the source of some problems, too.

[btp2k2]

I have painted all my MDF cabs with semi blacks and a coat of Killz primer and they look fine. I typically use Rustoleum Semi Gloss Black, but the last one was a latex based (Sorry, forgot the brand) so I could paint indoors....the finish came out looking just as good

[fjl]

I think for the perfect flat look, you'd have to use an air gun paint sprayer. With many wet sandings in between.

[Fozzy The Bear]

I think for the perfect flat look, you'd have to use an air gun paint sprayer. With many wet sandings in between.

NO!!  You just use the method I specified, with the thinned down waterproof PVA wood glue (yes that tip came from the bear).

If you want to see the quality of results you get from that go look at my control panel in the CP thread... In fact better than that here's a pic.

The main box is painted with only two coats of paint and no rubbing down between coats. It's extremely durable because the pva glue sealer makes a near perfect bond between the paint and the wood.

Method....
1)Water down some waterproof PVA white wood glue... about 70% water to 30% glue. Or maybe even 80% water.....
2) Paint it on to all surfaces and let it soak into the MDF.
3) Make sure it is completely dry. (Takes a day or so)
4) Sand lightly with 1200 grit wet or dry paper used dry.
5) Paint!!

Best Regards,
Julian (Fozzy The Bear) 

[needlesmcgirk]

Thanks for the help.  I think I have been shooting for the moon with the gloss, so I will defintely try again with some semi gloss.  Probably will re-prime the thing with some special mdf primer, but after priming it once, putting two coats on and sanding that, and two more coats and sanding again, I would really think the thing is sealed by now.  Any more suggestions would be great!

[crpngdth2001]

The main box is painted with only two coats of paint and no rubbing down between coats. It's extremely durable because the pva glue sealer makes a near perfect bond between the paint and the wood.

Did you use a foam roller or foam brush to apply the paint? I think I'll give this a try on the cabinet and control panel I am working on. Your results look great! Any other tips than thorough, even sanding and cleaning prior to starting?

[prOk]

Two things.. never apply gloss with a brush or roller, and never sand MDF unless you absolutely have to.

MDF can be 'polished' with 600 grit paper though.. that's the trick. get it shiny smooth again.

/brian

[Fozzy The Bear]

Did you use a foam roller or foam brush to apply the paint? I think I'll give  Any other tips than thorough, even sanding and cleaning prior to starting?

The front of the main box was done with a very close grained foam roller.... They sell them in the UK as a "Glossing Roller" and they give very smooth results, with gloss or mat paints.

As for sanding.... NEVER sand the mdf surface until after you have primed it with the above mentioned glue mix.  Also, because of the glue mix sealer you don't need to clean it down either. Just sand lightly with 1200 Grit after the glue mix has dried out.

I discovered this trick from a decorator who was having to re-paint the inside of a house after there had been a smallish fire in there. The Glue mix sealed in the remaining soot marks, so that they didn't leach through, into the new paint.

My uncle, who is a builder, also uses this mix to seal and smooth new plaster walls before applying paint to them.

Best Regards,
Julian (Fozzy The Bear)

[needlesmcgirk]

Why don't you want to sand mdf?

[javeryh]

I am in the process of finishing my MDF cabinet and here's what I did (against the advice of others here, I guess):

1.  Sanded the MDF with 600 grit sandpaper
2.  Applied 3 coats of oil-based white primer (sanding with 220 grit in between coats and waiting minimum 24 hours for each to dry fully)
3.  Applied 4 coats of latex top coat (sanding with 220 grit in between coats and waiting minimum 24 hours for each to dry fully)
4.  Applied 15 coats of spray on lacquer (no sanding in between and only waiting 20-30 minutes between coats)
5.  Wet sanded with 1500 grit sandpaper
6.  Apply 6 coats of automotive rubbing/cutting compound
7.  Apply 4 coats of automotive polish

It takes FOREVER but the results are an unbelievable piano-like finish.  I followed THIS guide to achieve the results.  I wish I could post a picture that adequately shows it off but so far I'm not having any luck with my digital camera.  Here's the before and after pics of the example I followed (speakers):

 

Amazing!

[fjl]

Is it reflecting the yard or something? Amazing!

Does it scratch easily and if it does scratch, is that easy to fix? Could you give us names of the products you used?

[Fozzy The Bear]

Why don't you want to sand mdf?

Because if you sand it first, you destroy the pressed surface finish that it has. MDF is made by taking pulp and glue and pressing it under hundreds of tons of compression between heated rollers. This gives it a smooth surface to start with. If you sand it you destroy that compressed surface and make it harder to get a finish on.

You then end up having to go through the sand and prime and sand and paint and sand and paint and sand and paint, process and days of frustratingly slow work that javeryh did. All be it with nice results, but also so unnecessary if you follow what I said above.

At the end of the day the choice is yours...... one days work, or ten days work and additional expense.

Best Regards,
Julian (Fozzy The Bear)

[needlesmcgirk]

Well, my paint job turned into a mega disaster.  I was sanding off the second coat, and I noticed it started pealing.  The entire paint job was ruined.  I'm chocking it up to the weather being a little too cold and me not waiting enough time to put the 2nd coat on.  I end up having to put some kind of goop on two sides of the thing and stripping all the paint and most of the original primer off and getting it pretty much down to the bare wood.  it was a very miserable and discouraging experience.

I bought some Killz Original and primed the thing, 3 coats this time.  I'm not messing around anymore.  I sanded between each coat.  I won't be able to put the top coat back on, but I have settled on semi-gloss as I thing straight up gloss was shooting for the moon.  Anyway, there's my update.  Hope it turns out better next time...