Undercar: The Science Behind Traction And Braking
Virtually all the refinish liquid of any kind that the collision shop buys is applied through a spray gun of some kind.
In a recent conversation with several old friends and industry veterans, the subject was spray guns and the many changes we’ve seen in our long tenures in the collision repair industry. I first wrote about spray guns for BodyShop Business in the September 1991 issue, a mere 28 years ago. What could possibly have changed? Well, both the technology involved and the cost of the guns have increased dramatically. But the underlying physics of atomization remain constant.
It is all thanks to Dr. Allen DeVilbiss, a civil war veteran who invented the atomizer to help medication stay in contact with his patients’ throats longer than if they simply swallowed the stuff. His son Tom successfully marketed their decorative cut-glass atomizers to dispense perfumes and then very cleverly adapted his dad’s idea to spray paint and varnish on wooden furniture in 1907. When the first 1923 Oakland was spray painted with nitrocellulose lacquer and a DeVilbiss siphon-feed spray gun, the car world changed forever.
I learned a ton about spray equipment from the DeVilbiss Automotive sales team in the late 1980s; they were the first “brand name” client on my resume. They wanted their sales folks to speak knowledgeably about collision repair in general, both the repair processes and the many versions of pulling and measuring equipment made by others. We did a series of classes together around the country, and I was honored to speak at their 100th anniversary celebration in 1988. (I still have the pewter commemorative plate on my office wall.)
Some of the things I wrote about in my 1991 BodyShop Business column remain applicable. The air cap on any spray gun still has three functions: S-A-P = siphon, atomization and pattern. Siphon or suction is created by crossing two streams of air until they create a partial vacuum and lower the 14.7 PSI atmospheric pressure that surrounds us all (what Dr. D discovered). Atomization is the moving air’s ability to break the fluid stream into ever smaller particles. And the pattern function both shapes the surface contact area and distributes the atomized particles within that proscribed shape. Back in the early 1990s, there was much confusion about fluid tip diameters as our industry switched almost overnight from sizing the holes in inches to using the metric system. For your review: 1/16” equals .0625 thousandths, which also equals 1.59 millimeters.
Virtually all the refinish liquid of any kind that the collision shop buys is applied through a spray gun of some kind. Historically, the painter provided their own guns while the shop bought the liquids. The shop typically owned only the beat-up primer guns that hung coated with drips on the masking machines until someone shook one of them a couple of times before applying the lacquer primer surfacer. These days, many smart shops are investing in shop-owned spray guns that can demonstrably save them money on paint and materials.
A single ounce of some basecoat colors can cost the shop $10, and the right spray gun, with the right tip and cap combination, can save their cost rapidly. If the new gun (with exactly the fluid/cap system recommended by your paint brand) costs the shop $700, they only need to save a little over two quarts of base color to recover the gun’s entire cost and save money each month going forward. The plan is to use today’s latest spray technology to conserve paint and material. Today’s ingenious engineering and machining manipulate fluid dynamics and combine air streams to increase atomization and delivery speed. It is sophisticated technology for sure. Using notched fluid tips, many little holes in the air cap horns and face (micro-piercings) and other ingenious aerodynamic principles, the current crop of refinishing spray guns are decades ahead of the guns I originally wrote about.
I asked one of my friends what happens when the prima donna painter insists he’ll use only his historic preference in spray guns. “Get a new painter” was his answer. We both agreed a good painter can make anything work, but as always in a body shop, labor time is the most expensive thing, much more than P&M costs. The only way to find out if a change in spray guns will save your shop money and/or time is to try some. Many PBE jobbers will offer their body shop customers a chance to paint cars for a week or so with their recommended guns and specific set-ups, with no obligation.
A Gun for Every Purpose
Lots of things affect the finish quality: in-flight solvent loss; consistent and correct droplet size; and differences in pigments, resins and solvents used by the various paint manufacturers. Waterborne won’t reflow like solvent-borne does. Sealer coats must be sprayed perfectly flat or the reflectants in the color may be thrown off-kilter, yet too fine an atomization can cloud up some high metallic colors. Numerous other S-A-P factors require just the right gun for every paint system to get the best results. I received expert advice about which guns and why from my knowledgeable friends.
Primer gun: Buying a new primer-surfacer gun with the latest in atomization technology and the exactly correct tip and cap combination will not only get to the required film build faster, but it will also save material too. And at a couple of hundred bucks a gallon for most any 2K primer, it will pay back quickly.
Sealer gun: Any texture in the sealer can affect the metallic angles in the color coat. You can’t sand the sealer flat to remove the texture as the scratches will break the sealer barrier. These days, gray-shaded sealers are part of the color match in addition to providing a uniform base and inter-coat adhesion. From light to dark with half a dozen gray values along the way, they all need to be sprayed perfectly flat. Many shops use an older, worn-out topcoat gun for sealer – it’s just sealer, right? Acquiring a spray gun specifically equipped for your brand of sealer will ensure the best reflectivity of the color coats.
Basecoat gun: Solvent or water both need the correct, and consistent, atomization to duplicate the OE coatings we’re all trying to match. Larger droplet size in waterborne finishes may equal better metallic control. Without solvents to reflow the film once it lands, the metallic is sometimes trapped where it hits. Too much atomization and fine metallic and pearls can look cloudy on horizontal surfaces. Ask your PBE jobber or paint rep for their specific gun/tip/cap suggestions for whichever product line you spray. They’ll have a recommendation they know works well with their unique chemistry.
Single-stage gun: No doubt similar in set-up to the clearcoat gun, the cleaning problem remains all these years later. Can you shoot solid colors through your clear gun? Yup, but the first time it isn’t spotlessly clean, trouble awaits – just like in 1991.
Clearcoat gun: Maybe even two clear guns, one for the “speed clear” your shop shoots and one for the “overall” version. The better the gun works, the less time polishing out flaws in the final coat. A clearcoat laid down nice and flat results in better reflectivity for sure.
Use today’s improved spray gun technology to conserve today’s evermore expensive refinish coatings. Should the painter be expected to spend $600 to $800 per gun, times four or five specific guns, to save the shop P&M costs? I don’t think so. I think the shop can more than justify the expense of multiple new guns when they, in fact, save money on material and atomize and layout each coating to the ideal finish.
One last painting comment from an old friend who was painting cars for several years before I joined our industry 49 years ago and is still at it. He said all his paint work sits on a scissor lift in the booth and he no longer goes up and down while he paints, the car does. Old guys rule!