Paint problem diagnosis

[gijoe985]

I could not find a paint section on the forum. I am a shop teacher and I have done my own painting in the past, but I am running into a little poblem that I have not seen before. Most likely because my students are doing something wrong that I have not done wrong before...

The paint is appearing to be "tearing." Almost like the paint is shrinking and small tears are appearing on the panel. (They are just practicing with sheetmetal panels.) After seeing a couple of students with this, I came up with one theory. Some panels it seemed that the paint looked good over the area that that they were doing their patching and bodo practice, but only the areas that were on the fringe, which they primered (oh yeah, this is happeneing with primer.) I think that maybe the tears are apprearing along sanding grooves that may have oil in them. Or at least it is an area that didn't get cleaned well before spraying. That's why it looks good ont he area they focussed on, but not the rest.

Still, it looks like tearing. Like the paint shrunk and it just tore at various points. Any tips would be appreciated.



EDIT-
Here's a great link on diagnosing problems, but I didn't see what my students are experimenting. Though a new suspection is that they might be putting a second layer of primer on with an incorrect flash time between coats.

Sherwin Williams Automotive Troubleshooting Guide

 

[Achelea]

Questions:
1.) What are you using for your cleaning?
2.) Do you have rubber gloves for them to use?
3.) What type of paint is it?

We used mineral spirits If I remember correctly to wash the sheet metal before we painted panels, and distilled water to rinse the cleaning agent off the panels and let them dry fully. That and when we painted we made sure to keep the time between coats a few hours. (Dunno, let them cure over night between coats?)

Then again I was using aviation methods and not automotive.

 

[Never Satisfied]

Sounds like they are not giving allowing for the full flash time between coats. Or maybe weather is not allowing the paint to flash as it should.

What kind of primer? What kind of paint?

I have never experienced this problem in the time that I have been painting.

Have you got any pictures of the problem that you could post?

 

[gijoe985]

I was going to take one on my phoine, but the kids sanded it off before I thought of it.

I'm actually just covering for this class as a long term sub, so it is my first day. I'll check on the type of primer.

They've got a state of the art paint booth, so I don't think that is the problems.

They are washing the panels under the faucet and then drying them and cleaning with a tack cloth before painting. I am starting to lean to not enough time between coats. We'll see... I'll try to get a picture too.



====================
Still no answer, but another good site. I just wanted to post it so Ic an refer to it later.

Frequently asked questions about urethane primers
And another
http://bobstory.tripod.com/faq.html

 

[SrKegler]

I've saw that is when painting over synthetic enamel. Try having them put on a sealer first.

 

[TiradoCC]

Clean, clean, clean.
If all they are doing is washing them and using a tack cloth, that is more than likely your problem.

Have them try this:

1. Wear nitrile gloves
2. Wash the panel with soap/water
3. Dry off
4. USE WAX & GREASE REMOVER, wipe on with 1 cloth and wipe off with another cloth.
5. Tack cloth
6. Spray primer

Since you mentioned a state of the art booth, I'm assuming you have a good water dessicant/oil filter.
Clean air, and a clean part right before spraying is the most important in my opinion.

 

[gijoe985]

Well, if it makes a difference, I looked and they were using Nason Acrylic Lacquer Primer. Some were using a high build and I don't think others were. Again, I am a long term sub for this teacher, so I am finding my way around. Today is day two for me.

As for the primer-

One of our suction fed cheapo guns had the primer totally harden inside of it. It was solid. A cleaning nightmare. While the second primer gun had it totally fin inside. Any ideas why? Maybe the one gun had been sitting for days and the kids just used the second gun rather than mention it to the teacher? The guns were full of primer when I got there.

How long would it take a properly mixed (big assumption there with HS kids) acrylic lacquer to turn completely solid? About 2" of solid paint at the bottom of the container, and solid all the way up the feed tube. Luckily we've got a really nice cleaning machine for guns...

EDIT- Just noticed that the paint that solidified was a urthane not a lacquer...