Paint Spray Guns

Non-Electrostatic

LIVE STORY: KEEP THEM HOSES SHORT

A while ago, I visited an aerospace facility where the painters were applying a high solids polyurethane topcoat to an aircraft using an Air Verter®  HVLP spray gun. Had the coating been high gloss, it would have looked awful on account of the heavy orange peel finish.  Moreover, the film thickness appeared to be extremely heavy, although I didn’t have my dry film thickness gage with me to confirm this. Had the coating been glossy, the painters would have been required to perform at least 40% rework.  This means unnecessary air and waste pollution, and regrettable, little or no profit for the paint shop!

The painters told me that the HVLP gun simply could not do a better job with this coating, and I offered them a friendly bet that the same gun could do a great job, if used properly. They had their doubts, but I had an ace up my sleeve that they could not have known about.  During the day I had been astonished by the length of their 3/8 inch diameter air hose, and now was my opportunity to do bring this to the light.  First, we strung out the hose in a straight line and I asked someone to bring a tape measure.  Despite even their guesses, the hose was 120 ft long.  We then attached a pressure gage to the end of the hose, and then attached Air Verter’s “eductor” and special air hose thereto.  The spray gun was attached to the end of the special air hose which is part of the gun design.  Like all HVLP guns, air pollution regulations require that the air pressure at the cap of the gun must not exceed 10 psig., but we were not even able to get to three psig.  I asked that the regulator mounted on the wall of the spray booth to be opened to its maximum, namely about 105 psig., but that made no difference.  The maximum pressure we could get at the end of the air hose, immediately before the attachment of the educator, was 20 psig.  This gun was certainly not designed for such a low input pressure.

The next strategy was to replace the 3/8 inch hose with a 5/8 inch diameter, and to limit its length to 50 ft.  When these modifications were made, all faces beamed as the painters witnessed the high solids coating laying down nice and smoothly, and with a decent dry film thickness.  No runs, sags, orange peel, or anything that might lead to rework.

The supervisor of the shop asked how he would be able to get all around the aircraft with only 50 ft of hose, and I suggested that instead of having only one or two pressure supplies on the one side of the spray booth, he would be well served to provide several compressed air off-takes at strategic locations around his large aircraft booth.

The lesson to be learned here is that no spray gun can atomize the average high solids coating when the pressure at the cap is so low.  In this case the painters were not equipped with pressure gages that would have forewarned them of the problem.  Fortunately, I carry gages with me and was able to demonstrate the cause quite easily. If your shop is also having problems atomizing high solids coatings you might want to check both the inlet and cap pressures of the gun. If you do not have sufficient pressure (what we really need is to have volume), don’t be surprised.  Keep the air hoses as short as possible, and if need be, purchase an air hose with a large diameter, such as 5/8 inch.  True, they are heavier and more cumbersome for the painters to drag around, but you will have a better opportunity to get the job done properly.

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