Paint Testing Techniques

COATING ADHESION WITH HVLP GUNS?

Q:  I wish to paint the exterior wooden siding of a residence using an HVLP spray gun.  There are some who argue that for good adhesion only a brush or roller should be used, while others say that spray guns give excellent adhesion results, and that HVLP is even superior to airless spray. What is your opinion?

Another reason for wanting to use an HVLP gun is that the overspray is less than for other spray guns.

W.D.

A:  The reason why a paint brush or roller might improve adhesion is because both methods give the paint a better opportunity to wet the surface and work the paint into the porosities of the wood siding.  These methods might be slower than some of the spray application methods, but they do work and have a proven track record.

If surface preparation has been "properly" performed spray application should also give you good adhesion, and thousands of homes and buildings that have been painted by spray application testify to this.  However, I am not in a position to tell you that spray application and brush/roller application give equal adhesion, because I have never seen any data on this subject.  With regard to the comment that adhesion with HVLP spray guns is superior to that for airless, I am skeptical because the high pressure of the airless application would, in my opinion, tend to drive the paint further into the fibers of the wood, thus benefitting adhesion.  Again, I haven’t seen any data on this topic, and so cannot give a definitive answer.

In terms of overspray, HVLP is good only when the gun has been properly set up.  Most people who work in a paint shop have observed countless situations in which overspray has been terrible because the atomizing air pressures have been set too high.  Therefore, I could not make a blanket statement that HVLP leads to less overspray when compared to airless, air‑assisted airless, electrostatic or conventional spray guns.  In fact, recently I observed a demonstration in which an HVLP gun was compared with an air-assisted airless device.  In that non-scientific and purely qualitative test the air-assisted airless gun appeared to produce less overspray than the HVLP gun.  There are many factors that affect overspray, most important of which are the manner in which the painter sets his/her air and fluid pressures, fan width and gun-target distance.  As I have written frequently in the past, transfer efficiency is often more dependent on the operator than on the spray device.

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