Build Your Skills: Composites (Part 11)

Part 11: Painting the beast. Understand the subtleties of methods and materials before trying them on the real thing.

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“Paint your vehicle with thinned Rustoleum and a roller!” Well, thats one bit of advice you’ll find on the web. At the other end of the spectrum is sending your several years worth of work to a distant shop, along with the deed to the house and your first-born child. How about a middle road? Say, do it yourself, but not with a roller? But not with a paint booth, either!

You can do it. Its not a slam dunk and will take a lot of work, but the satisfaction of getting a pretty fair job is enormous. Pride of parenthood covers a multitude of misgivings.

As the conclusion of our “Build Your Skills: Composites” series, this installment will help you make the final preparations for painting and, should you so choose, give you a basic grounding in the painting process itself. A word (or more) of warning: Painting is not inherently difficult but can be unforgiving. Take your time, learn all you can, and practice on small components or test articles before pointing your gun at your airplane.

Justification

Should you go for it? Well, the price of having a $40,000 airplane painted is about the same as having it done to a $400,000 airplane-typically $5000 to $9000 depending on location, shop experience and quality, and the amount of prep work left to be done.

Is it a good investment? For the $40,000 airplane you’ll spend 12% to 15% of the value of the airplane; for the $400,000 aircraft, less than 2%. The more expensive airplane will see the investment paid back when it comes time to sell. Regardless, many builders elect to paint on their own, for reasons of pride or economics.

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Before we go any further, you need to check your local environmental regulations regarding spray-painting at home. If you live in a condo, be aware that, even if not harmful, some of these concoctions lack only Macbeths three witches for being an evil-smelling brew. Painting at home is often not an option.

Tapping the Source

If you want an expert opinion, go see an expert, right? Joe Fields of Fields Aviation started by saying, “Preparation is everything. Shooting paint takes a bit of talent, but its not where the work is.” He pointed out an example of an aluminum aircraft with a beautiful paint job that, upon closer inspection, showed corrosion starting to lift the paint in spots. “Bad prep; you don’t have that problem with glass airplanes,” he said. “You have others.” What others? Why, pinholes of course.

Jerry Stimple owned the premier paint supply shop in the San Francisco area for 30 years and finally retired. He is a guy of unbounded generosity when it comes to helping his friends paint their “precious.” Hes done at least four in just the past couple of years and is able to draw upon all of the big paint manufacturers for the latest products. Hes not wedded to any one process or product.

Preparation

OK, back to the painting. In a previous article on composites I addressed pinholes, but in my research on this stage of the process, I kept being dragged back to the little monsters, so lets have another go at them.

Fields and Stimple both went to great lengths to give different approaches to the problem, so well reiterate. Fields recipe is to sand down to smooth “…but not through the gel coat! Itll only create more pinholes, and the fabric has gaps between the weave even if its resin-soaked, so the finished paint will show a pattern.” Stimple agreed, and joked that it would have more pinholes than the carpet in a tailors fitting room.

Both had an answer. Fields likes to start with a light scuffing with 409 cleanser and Scotch-Brite, followed by rinsing with clear water…at least 10 times. He emphasized that you should use the original, purple-label 409, not the later versions with aroma additives. The rinse is needed to get the 409 off, so don’t make it harder by adding Divine Springtime to the mix. Simple Green-Aircraft was also high on his list of acceptable cleaners.

Stimple prefers a tiny squeeze of dishwashing soap in a gallon of water, followed by a rinse mixture of 10% rubbing alcohol/90% water. He emphasized that a tiny squeeze is all thats needed for the same reasons Fields gave: You have to get it off. The alcohol/water helps, but don’t make it more difficult with too much soap.

They both agreed that automotive wax and grease removers (not to be confused with engine degreaser) are great on aluminum, but fiberglass tends to soak up the stuff and leave a film; before going this route make sure that whatever you choose leaves absolutely no residue. If you’re curious, put a couple of drops on a piece of window glass to see if it leaves a film when it evaporates. If you want to be a bit more aggressive, “Abrade the surface with a dual-action sander pushing 150-grit paper,” Stimple added.

The Shop Air System

Fields recommended finishing the cleaning process off with a high-pressure shot of air straight down on to the surface to blow off the dust. That brought up the subject of the air system, so lets digress for a moment into preparing the shop. Even if you don’t plan on shooting the paint yourself, if you have the air system wrong, you’ll set the stage for disaster when the color can is opened. (See the sidebar, “Compressor Come Clean,” Page 49.) With the hardware fixed, lets get back to the airplane.

Filling the Pinholes

You’ve washed the surfaces, and now you have to dry them. Heat lamps are OK, but that big heat lamp in the sky is better, especially if you’re painting parts rather than an assembled airplane. However you do it, be sure to do it, or you’ll trap moisture under the next coatings. “Paint on a glass airplane sticks by mechanically bonding with the surface, so scuff it and make sure its clean and dry,” sayeth Fields. He also advises using the product type that was used to build the airframe. “If the airplane is epoxy, don’t use polyester-based products.”

Stimple expanded on that idea and urged builders to, “Get to know your local paint supplier and rely on their expertise. Then buy the products of a single manufacturer; don’t mix them.” Among his product recommendations is Split-Second Rapid Sanding Glazing Putty from US Chemical. “Its so lightweight you’ll think the can is empty.”

In my previous article on filling pinholes, my sources were adamant that pinhole-filler should be applied as thinly as possible, but Stimple said unequivocally that Split-Second could be slathered on up to 1/16 inch thick and then sanded to smooth out ridges as well as fill pinholes. The other sources didn’t mention this product, but both groups got fabulous results, so it must be the product.

Once it sets, you can sand it down followed by a wipe with a clean rag lightly soaked with alcohol. The alcohol evaporates, hastening the drying time.

Fields emphasized that whatever product you use, it should be both lightweight and flexible. “Ive seen a lot of airplanes that looked great until after a few flights caused hairline cracks at the wingroot,” he said. “In fact, I prefer to leave a floating gap like a freeway expansion joint.”

In the previous installment, we spoke of a guide-coat, and both Stimple and Fields agreed that it was a good idea, so Ill reprise it here. When you think you have a smooth surface, clean it and spray it with a high-contrast paint. A rattle-can is fine. Then, sand that off using a long board to expose low spots. Don’t use a short block or, worse still, your hand to back up the paper. Either of those will just follow the curves. You may have to do this a couple of times to get that jaw-dropping, mirror-flat finish. Skip this step, and you’ll have an adequate, but not spectacular, result.

Lighting

Heres an area where most of us go wrong: We add a bunch of shop lights up high over the project and consider it adequate. Not according to Fields. He strongly recommends that you set a 4-foot fluorescent lamp on a tripod and position it at the height of the surface you’re working on. At the other end you should have a dark background. The combination, he says, makes discontinuities show up faster than the end of a three-day vacation.

Sandpaper

All sandpaper is not alike. The cheap stuff is likely to have varying sizes of particles, and you’ll only create scratches while trying to eliminate them. Buy it from your local automotive paint store, not the hardware store; hardware customers are sanding on wood and laing down heavy coats with a brush.

The Spray Gun

If you’re like me, you’re hoping that a better gun will offset the need to sand, sand, sand. It wont. But with that desire in mind, Ive saved the spray gun for last. You’ve cleaned up the air delivery system, so lets talk about the business end.

Now hear this! A compressor, 50 feet of inch line and a $25 spray gun is not adequate. If you cant or wont build a good air system, stop right now and send your project out to be painted.

Lets take a look at spray guns and work back toward the compressor. There are lots of ways of spraying paint onto your airplane, but that airless sprayer you used on the house is not one of them. The three most common ways of spraying paint are high-pressure guns, turbine systems and HVLP guns.

High-pressure guns. These are the cheapies that operate at about 45 to 60 psi and 8 cfm. Paint has become expensive, and the EPA put a halt to some of the really nasty chemistries-especially to the quantity of solvents that were being dispersed-so these guns are relegated to water-based paints.

That led to turbine guns. These use a small turbine blowing warm air through a large diameter hose to generate 3 to 8 psi and 100 cfm. The first versions of these were built using vacuum cleaner components, and they worked well. The low pressure means that they gently lay the paint down instead of blasting it all over the shop, so, with practice, you can get as much as 85% transfer efficiency. With paint costing $200 to $400 per gallon, thats an important number. To learn more about these, visit www.fuji-spray.com/what_is_hvlp.html or http://www.turbinaire.com/.

Those vacuum cleaner parts led the high-pressure gun guys to figure out how to do it without the turbine. Although a turbine and an HVLP system work to produce a gentle spray, they have different approaches and advantages. The turbine doesn’t overly compress and then expand the air, hence theres no entrained condensation, and what little heating there is tends to aid the atomization of the paint. Forget about entrained oil. Point scored for turbines.

Getting set up with HVLP is cheaper, though, assuming you already have an adequate compressor with dryers and traps for your other air tools, all you need is 3/8 inch ID or larger air hose and a gun. A turbine system goes for about $800, while a top quality HVLP gun is less than half that. Point for HVLP. Those $50 HVLP guns? Stimple wont touch them. Visit http://www.binks.com/   to see the gold standard in this area. By the way, you should have those dryers and traps even if you don’t have HVLP. How do the experts vote? Fields likes his turbine, and Stimple likes his HVLP.

Stimple then tackled the setup of the HVLP and cleared up some misconceptions. HVLP does not mean low-pressure air is supplied to the gun. “The gun itself reduces the pressure,” he said. “You need about 45 psi at the gun, and the line loss is about 1 psi per 10 feet of 3/8inch hose. So, for a 50-foot hose, crank up the pressure to 50 psi and let the gun do the conversion from pressure to volume.”

He also described a “cheater,” a small gauge with a valve that can be attached to the base of the gun. “Dont bother with them,” he said. “They reduce the pressure but don’t regulate it, so as soon as you let off the trigger the line pressure goes back up to tank pressure. Just set it at the tank where you’ve got a true regulator.” Of course, a portable compressor used to drive a nail gun wont keep up with an HVLP gun, so when you’re shopping for a gun, be sure to check that requirement.

As for gun configuration, there are some with one-quart cups on top, with a cup on bottom, and others with a siphon tube leading to a separate paint container. Stimple says that a top cup has the advantage of gravity feed and may, therefore, require less pressure and use 100% of the paint.

If you have a gravity feed cup, shooting upward onto the bottom of the wing is nigh-on impossible. 3M came up with a dandy accessory, though. Its a bag that, once filled with paint, fits in the bucket. You invert the gun so that all the air in the bag is blown out; then it matters not what angle you’re shooting.

The turbine gun is a bit different. Read the manual, plug it into 110 VAC, and go spray.

Protective Clothing

The EPA may have gotten rid of the worst of the concoctions, but you should still not go cheap on a respirator. Heres one good reason to get to know your paint supplier. If he/she advises a full suit and external air, either buy it or change paints. You don’t want the paint job to last longer than you do.

Lets Start Painting: The Primer

Two-part primers were recommended by both of our experts. Better adhesion with a smoother surface was the advantage. But check with the manufacturer of the final paint system to ensure compatibility. Sand the surface lightly with 180 grit, blow it off and then shoot it again. Then give it a light sanding with 400 grit and water, and let it dry.

Take your time here. Be sure there are no visible flaws after the final surface sanding or theyll show through the final coat of gloss paint. When in doubt, sand smooth and re-prime.

Finish Coat

Paint schemes and colors are totally subjective, except that most composite-kit manufacturers specify white (or at least light colored) hues to reduce temperature buildup in direct sunlight. Moreover, our experts said to avoid metal-flake paints; many of them contain copious quantities of aluminum that will interfere with internal antennas.

Fields advises that, if possible, paint the wings separately from the fuselage. Two good reasons include the obvious fact that doing it that way is much easier than shooting straight up to the bottom of a wing; its also easier to control overspray. But if you cant, cover the wings with bed sheets and spray the fuselage from nose to tail. Before you do that, look for obvious start/stop points such as seams, and use a fine-line tape to tape off what you’re not painting. When that area has set up in 24 hours or so, you can switch over to painting whats still bare. “Dont spray over the overspray!” Fields warns. “Pass your clean hand over a surface and if it makes a rough noise, use a fine Scotch-Brite pad to smooth it.”

Layout of a multi-color system has its own perils. Stimple advises that if you can get your hands on one of those old overhead projectors, put a half-black sheet on it and you’ll get a perfectly straight line on the side of the fuselage. The paint job will also require a clear-coat to blend the ridges at the color interfaces. Therefore, each color should be thin, and that requires a very good base or any less-than-perfect sanding will show through.

Now, heres the crazy part: These two-part paints are made to be buffed to a glass-like smooth surface, but to keep the weight down we have to keep the thickness down. When we put our final clear coat on and buff it down to level, the scratches in the color show through. That sounds like a fools errand, but Stimples Lancair is a four-color job with a clear coat that is unbelievably flat and glossy. “Its all in the sanding” he says. “If thats smooth, the colors will be smooth.” He also unequivocally recommends the 3M product line. “You cant get in trouble,” he says. (Im going to test that bet and will report the result.) Fields favorite supplier is Poly-Fiber.

The color coat, if its the final one, and the clear coat require buffing. But don’t use anything larger than a 7-inch wool or foam pad at 1200 rpm. “Do not use a 90 grinder with a pad; you’ll burn the paint!” Stimple says. One last tip: You could wet the floor down to suppress dust, but a better idea is to use Sherwin-Williams Dust Free #55. Spray the walls and floor, and it will keep down the dust by adhesion and electrostatic attraction.

Painting Outside

Stimple has done several aircraft right out between the hangars and within 100 yards of a freeway with astonishing results. “First, understand that dirt is unavoidable and that these new paints are, as Ive said, designed to be buffed,” he explained. “Still, you should pick a calm-air day, early in the morning, and with a heavy dew fall to suppress the dust. 70 F is ideal, but if its cold or windy, forget it.” Incredulity alternates with skepticism until one sees the result. Then astonishment kicks in.

Vinyl Striping

If you’re going for a bit less complex process, Stimple highly endorses the use of vinyl stripes and overlays. These allow you to paint one or two colors that cover the entire aircraft, and then add the accent color where you want it. They are usually computer-cut, so almost any design is doable. Those jazzy wave-like patterns on a Cessna 172 are a good example. “Just make sure the surface is smooth, because the tape will magnify every bump,” he says. (Applying these is a topic in itself that Ill cover when I get to that stage with my own project.)

Is that all you need to know? Although there are books on the subject, both large and small, some near to “Dummies Guide” level and many that are cures for insomnia, theres no substitute for practice and patience in doing the prep work. As a Lancair owner said, “Everyone has a fixed quantity of sanding strokes built into them. When you reach your quantity, you’re done.”

So procrastinate later. Get a big box of sandpaper, a good spray gun, and remember the words of Dizzy Dean: “It aint bragging if you done it.”

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