Ask the Expert Question-and-Answer Archive

by Ron Joseph

June, 2005

Spray Applying High Solids Coatings--Thinning

Q. There is a good deal of speculation today about the development of paints (especially Chemical Agent Resisitant 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 that of water-reducible paints, that can be sprayed out of the can? We used a well known HVLP gun with about 35 pounds 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 fluid ounces/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, which as you know is an exempt solvent.

I hope I've answered your question, otherwise please get back to me again.

Best wishes,

Ron Joseph

Q2. Many thanks for your cordial and very helpful advice. We all doing well, with plenty of work, thanks be to God. I have great curiosity about the viscosity established by the manufacturer for some of the paints that can be sprayed 'out of the can'. I wonder if they actually contain added solvents to allow spraying rather than to be added later (at 20% by volume), as we do. We try to keep our pots at 35 psi but know that with 25' of hose, the pressure to the gun is a lot less. I have gauges but did not put one on the gun we used in the experiment (a Binks Mach 1, HVLP) . We want no more than 5 or 6 psi at the nozzle. Do you think that to be enough?

A2. If you are using a pressure pot with a single regulator then the air atomizing pressure to the gun is less than 35 psig because of the pressure drop through the hose. The fluid pressure, however is too high. I suggest that you install a second regulator on the pot; the one which applies pressure inside the pot can be set at 10-15 psig, and the other will regulate the atomizing air. Regulate the air pressure from the second regulator until the pressure at the orifice of the HVLP paint spray gun, when the fan is set just the way you like it, is at 10 psig.

Please let me know if this strategy works.