Fine Finish Spraying: Not a High Pressure SituationPosted on September 25th, 2015 By Scott Burt No comments
Sprayer selection for finishing projects is really a simple matter, which has only become complicated by the fantasy of having one type of tool technology that can do it all. This dream doesn’t just apply to sprayers, but all tool types in general. Everyone wants to find the one tool that can “do it all”. It’s a nice thought, but really doesn’t coincide with the realities of finishing.
Because I deal mostly in the realm of professional paint contracting, I see a lot of cases where people try to drag that dream into existence by misusing that archaic weapon of mass atomization called the airless sprayer. If a painter has just one sprayer, it is most likely an airless pump, and they try to do everything with it. There was a time when you just about could, but that time has passed.
In the past decade, paint products have changed dramatically. Because of heightened VOC regulations, we have all witnessed the long drawn out swing of the formulation pendulum back around to waterborne finishes, and away from oil/solvent based products. As a result, the fine finish limitations of the airless sprayer have become glaringly apparent in the realm of fine finishing. This time around, the waterborne finishes are really good, viable replacements for oil. They are here to stay, which leaves many a finisher struggling with the failure of applying outdated tools and processes to new finish technologies.
Specifically, painters have an odd tendency to spray .5 gpm (or higher) sprayers wide open at 3300 psi (or more) on interior ceilings and walls, or even exterior facades, in production mode. This has long been a benchmark or “standard” for spraying, because it produces results on large, open road tasks. But then, there is the odd desire to turn right around, put a “fine finish” tip on the same airless machine and want so badly to believe in a magical transformation into a cabinet grade finishing sprayer.
What Happens When you put a Fine Finish Tip on an Airless Sprayer?
It would be nice, but real life doesn’t really work that way, and neither do sprayers. The old “airless with fine finish tip” trick kind of worked ten years ago when products were different. Oil trim paints were thinner, stayed wet longer and could be run through airless sprayers at reduced enough fluid pressures to do small tasks fast and get out of the space before the overspray/airborne mist gremlins set in to compromise the finish. Oils were thinner and easier to spray. With the new generation of waterborne finishes, a fine finish tip on a low transfer efficiency gun tied to a large pump results in neither efficiency nor high quality finishes.
It is the nature of the airless sprayer to require high fluid pressures, and to “shear” the material, resulting in a coarse atomization in comparison to the finer fluid droplets delivered by HVLP turbine units. This directly impacts the way products “lay down.” Add a roomful of airborne mist settling into the wet finish, and airless results become even more coarse. My own professional observation has been that the smaller the orifice of the “fine finish” tip on an airless sprayer, the more the fluid is sheared during atomization. This just doesn’t result in a fine finish on any type of scale, because latex doesn’t shear well. At least not well enough to meet generally accepted fine finishing standards.
The bottom line on the airless/fine finish tip myth is that painters who attempt cabinet grade finishes with this set up are for the most part plagued by inefficiency and overspray due to the poor atomization that results in airborne mist settling in the wet finish as it attempts to lay down, in turn yielding a less than smooth finish.
In fact, the larger the fine finish project, the worse the result usually gets when running an inefficient technology. And, of course, the act of just setting up a typical airless requires about a gallon of paint just to fill up the 50’ of hose and the piston pump.
Get More Quality “Bang” Out of a Bucket of Finish
As noted above, with the paint industry shift to waterborne finishes, my paint company has adapted its small scale fine finish program around HVLP, for a number of reasons. While waterborne finishes require high fluid pressure to pass through a small orifice in an airless machine, they happen to respond very well to high velocity and lower pressure (HVLP) spraying.
In very simple and practical terms, I (or just about anyone), can produce a 6” fan by running a 5 stage turbine at 9.5 psi through an HVLP gun that will produce a flawless cabinet grade finish. Meanwhile, a 6” fan through an airless machine requires about 3000 psi, a whole lot more material and yields a poor result.
A More Efficient Delivery
HVLP systems routinely deliver materials (conservatively) in the upper 80% range, 86-90% transfer efficiency seems to be the standard range of quality turbine systems. Airless guns are so much lower by comparison that manufacturers generally don’t share airless gun transfer efficiencies, focusing instead on the production value of the technology. While airless are solid production machines, it is a bit of a guessing game as to where in the 50-70% efficiency range they might actually measure.
In three decades of spraying airless systems professionally, there have been many times when I have personally observed as much material landing on the floor and on the operator as on the walls or ceilings being sprayed.
By way of quick summary, the obvious advantages of HVLP over airless for fine finishing include:
- Less materials consumed
- Quicker set up and breakdown
- Better quality finish
Misconceptions About HVLP
In my opinion, and this ties back to the desire of all consumers these days to find “the one that can do it all,” HVLP is sometimes overlooked by finishers because it is perceived as a specialist. And, to an extent, in the big picture of painting, it is. You probably won’t paint an entire home interior with an HVLP. Large scale production of many thousands of square feet at a time is not its strength. That’s fine, there are other machines built for exactly that.
However, it is possible to find enough variety of finishing tasks for an HVLP to become a general solution to a whole range of specialized task types that would otherwise be best suited to manual (brush and roller) application. HVLP shines in projects that are too small, complex or detailed for larger technologies, and too large or intricate to attempt manually by brush.
A great example of this is exterior spindle and rail systems on decks and porches. We have done many new spindle/rail systems in our shop and just as many onsite in exterior repaint situations. Another example would be a small quantity of interior or exterior doors. Take a quantity of 5-6 interior doors. Not worth setting up a pump and hose style sprayer, but quick and easy with an HVLP system. Custom cabinetry and millwork applications are another specialty.
The bulk of this discussion has centered around using HVLP as a more qualified replacement for airless sprayers in waterborne primer and paint grade situations for fine finish applications. One last strength I would mention is the absolute mastery that HVLP demonstrates in waterborne clear finishes. This is a realm where airless spraying is mostly incompetent, because airless pumps just aren’t happy operating at low pressures in thin viscosity materials.
So, it is possible to “have it all”, at least within the discipline of small scale fine finishing, and HVLP is the solution.
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