Ask the Expert Question-and-Answer Archive

by Ron Joseph

February, 2004

Epoxy Paint with Cold Tar Pitch

Q. I have a problem with above mentioned paint. It is connected with pin holing. I tried many different air release or defoaming agents and the problem still exists.

Application by brush gives quite good effects, but hydrodynamic(airless) application is more important for me.

Trying this method I obtain surface with pin holes.

I use two epoxy resins. First one contains coal tar pitch, and I don't want to change anything in it. So the second one contains all pigments, additives and so on. I use poliamide curing agent. So I have a three component product. I can't change it becouse we buy epoxy resin with coal tar pitch and we only want to put it in smaller containers and do nothing more with it. Maybe to many components is the problem? (difficulty with air release after mixing all together). Or maybe I should use some special additives/components that are of crucial importance in this kind of paint - I have no experience with it. If yopu have got any suggestions please answer as soon as possible. I have been trying to solve this problem for a few months now, and I haven't got the foggiest idea what else I can do to succeed.

A. Pinholing results when too much paint is applied too quickly and the solvents are not given sufficient time to evaporate. Alternatively, pinholing can result if some of the solvents in the paint film evaporate quickly, while others remain even after the coating has started to form a film that blocks the release of the slower evaporating solvents.

Consider the solvent balance of your coating. You always need to have some solvents that evporate quickly, some that take a little longer, and solvents that are released last. It is possible that the solvents in your specific formulation are not properly balanced, and don't expect the solvent balance of your coal tar epoxy to be the same as your pigmented coating. If a change in formulation doesn't work, then you need to look at the application.

Perhaps your fluid pressure is too high and you are forcing too much coating out of the spray gun too fast. Alternatively, the orifice of your airless spray gun might be too large. Try using an orifice with a smaller diameter. By applying thinner coatings all the solvent can evaporate before the coating starts to form an impermeable film. Recommend that your customers use an electric fan to move air over the coating. This is not the ideal solution, but it will help to force the solvents out quicker.

If you simply can't solve this problem we will be happy to assist you, but we will charge a consulting fee for our services. You would need to send us some of the coating so that we can apply it ourselves with an airless spray gun.

Good luck and please let me know if and how you manage to solve the problem.