Pin-Holing on Paint and Dust Control Issues

One possibility: If the workers are not wearing gloves when they sand and are touching the substrate, the oils from they're hands are getting on the metal and causing the urethane topcoat to separate. Another possibility is insufficient dry time between coats. Even if the surface is dry enough to sand, if there are solvents still trapped in the previous coat and you apply your top coat, those solvents will find a way to escape and pop through the topcoat, hence the pin holes. Hope this helps, if I can think of anything else I'll let you know.

GA. The outgassing is the most likely culprit, and for some reason the pinholes will persist even after a re-coat. Is that your experience?

Since the beams are not moved between operations I would suggest turning your spray booth into an autoclave between coatings. Any 2 part coat needs more time to cure than just being 'dry'. After the coating is 'set', slowly raise the temperature in the booth to about 150f, 180f maximum (see mfrs. recommendation). 8 hours should be sufficient. Maybe less temp, heat soak. Experiment with this. (These figures are from memory of old type resins, may be faulty)

Each coating should be pressure washed after cure, (removes amine blush, and this too may be the culprit) so put drains in the floor, coat the walls with a water resistant paint, and protect electrical equipment. This allows you to skip dry sanding. instead use an abrasive cloth (Scotchbrite® purple) and some 'wet' water and elbow grease. Flush with potable water. Dry and go to the next coating. Works for me

Yes the pin holing will persist with recoat, especially if spraying, simply because you can't paint a hole. If they attempted to fill the voids with a spray gun, they would just create massive runs. I suspect that since they are using airless guns and painting girders with lots of angles, they are also getting to much mill thickness where they overlap and it's possible that the out gassing is not from the previous coat, but from where the paint is so thick that the surface is drying before what's underneath. I recommended this on another thread, regarding the painting of metal. This is another case where electrostatic painting would be ideal. They would save a lot of material and since the paint actually wraps around the corners and edges, they would be able to control their mill thickness much better. I personally would split that final urethane topcoat into two thinner coats. If the last coat is applied within a certain amount of time no sanding will be necessary between coats.

One more thought, the original poster didn't specify where the pinholes were occurring, but all of the welded spots are notorious for creating pinholes due to air bubbles in the welds. I would wire brush all welded areas and apply that first primer coat to the welds with a brush, working it into the voids and creating a good substrate for the subsequent sprayed coats.

Yup! GA. I used to make electrostatic supply units for Nordson, so long ago that I forgot about it. What do you think about wet sanding/scuffing and wash down?

Sounds like a big mess. Also if there are any voids in the original primer, even invisible ones, they will be creating a situation where rust can take hold within a very short period of time. That's why I was thinking denatured alcohol. If these things are so big that they can't be moved from the spray area, I think introducing water would be a nightmare.

"you can't paint a hole"

I hope there's a time soon when I can use those words, with your permission.

I'd never exactly thought of it in those literal terms.

Feel free to use it whenever you want. It's a phrase that knows no boundaries, use it at your local bar, weddings, etc.... The reason you can't paint a hole is because every pinhole is like a miniature volcano, the gas escapes and leaves a little hole, beneath that little hole is an even bigger cavity.

Surface contamination i.e., sanding dust could be a factor, but would more likely cause fisheye as opposed to pin holes. If you think that may be a factor do a quick wipe down with denatured alcohol and lint free rags. Don't use tack rags, they are impregnated with polyurethane which is incompatible with your other coatings.

"CFO thinks air quality might be part of the problem so she" Two strikes against you, already.

If you truly have pinholes, the likely cause has already been addressed. Gas under the top coat.

Are the pinholes worse where there are fillets (thicker material)? Or in the flat?

Did this problem come with the new spray booth?

Have you run test panels along with the product? Something of known cleanliness about 12x12. If you can get the pinholes in your test panels, send that to the paint guys.

Not much help, I know.

Is there any silicone spray in your factory? It can migrate for vast distances, but would cause reticulation (fish eye) not real pinholes.

Good luck!

If you really want to know, I think you'll want a consultant that has the support of a good lab. I'm not associated with them any more, but I used to work for KTA-Tator in Pittsburgh, PA. They know bridge coatings and coating failures "in spades" and I'm sure that they can help.

One more thought, these things sound like they're pretty big. Are you using any type of reducer in your urethane topcoat? If so I would switch to a slow drying reducer in order to maintain a wet edge. That urethane fires off pretty quickly and if you're going over areas that have already started to set up, that too can create problems. i.e gassing off, not of the previous coat, but of the coat you are currently applying, by going back over areas that have started to set up.

laminate it with platic thin sheets by heat guns after wrapping ,