- 08 December 2005 -
Painting Problem Solver: Spray Applying High Solid Coatings
by Ron Joseph, Organic Coatings Editor

Q: There is a good deal of speculation today about the development of paints, epecially chemical agent resistant coatings (CARC), that do not need thinners. I have tried this and the finish is a dry dusty coat that is easily rubbed off. Have you any experience, other than water-reducible paints, that can be sprayed out of the can? We used a well-known HVLP gun with about 35 lbs. of pot pressure. If you know of paints being sprayed without thinner, what equipment was used?

A: I’m sure I have seen high solids coatings sprayed out of the can without thinning, but since I’m not the one who mixes the paint I can’t tell you for sure. I do know for sure that coatings at 3.5 lbs./gal. can be spray applied using HVLP guns. If the paint you are using is at 3.5 lbs./gal. and you are not able to atomize it, there is a strong possibility that the fluid pressure at the gun is too high and you don’t have sufficient air to atomize it.

Why do you need 35 psig pot pressure? Most painters like to adjust the pot pressure to high values, but they are unaware that high atomizing air pressures are required to break up the paint. Have you measured your fluid flow rate (mL/min., or fl. oz./min.)? That is where you need to start.

Also, measure the air pressure at the handle of the spray gun. You might find that despite having a high pressure at the regulator on the wall of the spray booth, the pressure differential in the air hose might be so high that you don’t get sufficient air to the gun.

Perhaps you’ve already considered these options. If you find that you absolutely need to add thinners but are at the regulatory limit, then consider adding PCBTF (Oxsol 100), which is an exempt solvent.

Lifting or Peeling of Lacquers
Q: There is a problem while painting hammertone finishes. I used to coat air compressor bodies in air drying hammertone finishes in shades of green and light blue. The paint system I follow is: N/C-based grey primer, allow to dry for four to six hours, apply air-drying hammertone finishes, keep drying for four to six hours, and then on to further assembly.

A few days after assembly, the lacquer starts to peel, especially at the outlet end of compressor body. There is also a loss of hammertone.

A: This question has been answered by my consultant colleague, Jim Burke. Unfortunately without seeing exactly what is happening, I can’t help you. An important question that needs to be addressed regards the type of coating system you are using.

Some fast-drying “lacquers,” which are more accurately called acrylic-modified alkyds, lift when the intercoating time is in a critical range. This usually does not occur with true N/C lacquers, but perhaps there is something in the formulation of your lacquer that results in this lifting defect.

Try changing the intercoating time between primer and topcoat to less than four hours (one to three hours) and also to more than six hours (eight to 12 hours). This might solve the problem.

Surface contamination could be a problem, since you find the peeling only at the compressor outlet. Are you testing the compressor at any time before or after the coating has been applied? What is the temperature of the surfaces where the peeling is taking place? Is it possible that after assembly the compressor is tested and oils or other contaminants in the air deposit on the surfaces that you previously painted?

Lifting or Powder Coating on Aluminum Railings
Q: I am a general contractor located in South Carolina. I had aluminum railings manufactured and powder coated for a project consisting of several thousand feet of walkways. The building is located directly on the ocean front and after three and a half years the coating is lifting at all joints. There appears to be a primer or some kind of treatment under the coating that is not lifting. My questions are: Do you consider this to be normal for a powder coating system? Do you have any idea what would cause the lifting? Is there a way to repair these failures in the field? Can an aliphatic polyurethane be applied over the powder coating? Any help you can offer would be greatly appreciated.

A: In my opinion, the powder coating should not have failed within three and a half years, but this could have been influenced if the railings were poorly handled during installation. If mechanical damage was minimal, I would not expect this type of failure so soon.

Several options come to mind. The failures can be due to inadequate surface preparation, improper selection of the primer, and/or improper selection of the powder coating for the marine environment, and/or improper application of the powder coating by the fabricator/powder coater. I am currently working on a similar legal case where exactly the same issues are being explored.

If the powder coating system is poor, you should remove it and start all over again. If most of the powder coating is still in good condition and if it looks as if the railings are likely to remain in a good condition, then you should scrape off all of the loose and flaking powder. You can paint over the intact powder, but you will first need to prepare the surfaces by sanding and solvent cleaning. Apply a liquid epoxy primer followed by a liquid polyurethane. It is possible that a single coat of polyurethane (absent the epoxy primer) will work, but you will need to experiment. Polyurethanes can be applied electrostatically, and perhaps this is the best method of application.

Here are some words of caution: You must take all necessary health and safety precautions to protect yourself (your painters) and the residents from inhaling the polyurethane vapors. If nothing else, those who are applying the coatings must wear the appropriate respirators. Your paint supplier should be able to recommend the proper personal protective equipment. Warn all the residents that you will be using polyurethane and strongly, very strongly, demand that they leave their apartments for several hours during the painting process.

Some people are highly sensitive to unreacted isocyanate vapors from polyurethanes, and as a consequence they could have serious breathing problems

In addition, I suggest that you take precautions not to get paint overspray on the cars and other property of residents, otherwise, you will have a nightmare legal problem on your hands.

Ron Joseph is an independent coating consultant in San Jose, Calif. You can e-mail questions to drrojo@aol.com.