Hands On: Spray Painting

We look at prep and paint application using Loehle Aero Coatings.


Last month we talked about how the Loehle process was developed and some of the preparation required prior to applying paint. This time we’ll actually do some painting using these products and methods.

Microballoons are mixed into the Filler/UV blocker before it is applied to fill in blemishes.

First, some safety tips. When spraying any chemicals, use proper skin, lung and eye protection. Good coveralls and gloves are standard equipment for all painters. A proper charcoal-cartridge type respirator is the minimum one should use when spraying paint. If you have access to a fresh-air respirator, that is even better. You should cover your head and eyes. Goggles with tear-off plastic sheets are available.

Getting Ready to Spray

Applying the first spray coat of Black Filler/UV Blocker is easy. We use spray guns, referred to as primer guns, with tip sizes of 1.7 to 1.8. Before spraying, blow off the part with clean, compressed air. We use a clean latex glove on one hand as the air is applied from the blow nozzle with the other. The glove aids in breaking dust particles away from the part surface because it cuts the surface tension and static, and air blows the particles away. Be careful not to get the air so close to the surface that it blows out the Wonder-Fil you’ve already applied. And do not use surface cleaner at this point because it also will remove the Wonder-Fil. A tack rag can be used instead of the latex glove.

The microballoon/filler-blocker putty takes longer to dry than a spray on, so accelerated thinner should be used to speed up the process.

The mixing ratio for our black and white blockers is 4:1:1 or four parts blocker, one part Universal Thinner and one part Filler/UV Blocker Catalyst. We use marked, graduated mixing cups to measure quantities for large parts. For small parts, use small mixing ladles. The ladle method is really accurate and avoids wasting chemicals. There is a short learning curve to figure out the amounts needed for various part sizes, but you will pick it up quickly.

The air pressure used to apply the blocker is 6 to 9 psi at the cap for HVLP guns and 25 to 45 psi for HVLP and siphon-feed guns. (The 25 to 45 psi is at the incoming end of the gun and with the trigger pulled, not the gun at rest.) The more air used, the better liquid chemicals are atomized. When you hear of “orange peel” painted surfaces, generally they start out with the air pressure being too low. Modern HVLP spray guns are notorious for this. (An article on the Loehle web site discusses spray gun tips.)

Here the putty mixture has been applied to a wheelpant.

Another way to reduce the chance of orange peel is with the thinner. Extra thinner gets paint to flow nicely, but you lose the ability to cover in as few coats as possible. Older style and cheaper paints use extra thinner in their products because thick, high-build chemicals and pigments cost more.

We prefer to change the temperature range of the Universal Thinners we add, and this step changes how fast the paint will “flash off” or dry. The longer it takes to flash off, the more time the paint droplets have to flow out evenly. Too much added thinner or too slow a flash rate will contribute to runs.

Spraying primer is a great place to learn how to paint, so don’t lose sleep over it. The Loehle system was set up for novice painters as well as pros.

The first coat should be applied by testing the spray gun on a clean piece of cardboard or masking paper taped to the wall. This is standard shop procedure for most painters.

I like the spray fan to be slightly under the widest fan pattern. Remember that when you narrow the fan spray pattern, paint is being applied to a tighter area, and runs can quickly show up. In primer especially, you just sand them away.

Once all blemishes are addressed, you’re ready to move on to the next step.

Spraying the First Coat of Blocker

After setting up your spray gun, spray one full coat of the black blocker onto the part surface. Move the gun slowly enough to completely cover the surface as you go. Too fast, and the paint will be thin. Too slow and, you guessed it, runs.

The White Filler/UV Blocker provides a nice base for bright colors.

Some painters will want to apply two or more thinner coats instead of one thick one, and that’s OK. But what I’m trying to illustrate here is the coverage you get from one coat and how it allows you to get used to spraying. The black sprayed over a composite part easily shows you exactly where you’ve painted and any spots you may have missed.

Allow the part to dry well before you sand. This can vary from 5 minutes to an hour depending on the temperature of your shop and which thinner you chose. Setting the part into direct sunlight will speed up the drying. When dry sanding, if the white dry sandpaper gums up, the primer is still a little too green.

Sand the part with dry 150-grit open-coat sandpaper to see the blemishes and pinholes remaining. You can use a 6-inch orbital DA (dual action) sander, but the parts can also be sanded by hand.

Gray primer can be hard to cover with some colors, so white primer is preferred.

The blemishes will show up immediately. Missed pinholes can have Wonder-Fil applied to them and large areas can have epoxy or polyester filler added to them. The epoxies may slow the process of filling and sanding by a whole day. I use mainly polyester fillers but might choose epoxies when working with the glass on top of an RV-10.

Mix microballoons directly into the Filler/UV Blockers and putty these into blemishes. This eliminates any “foreign” fillers being introduced to the primed area but will require a longer dry-to-sand time than a sprayed-on chemical. Accelerated Thinner should be used to help speed up the drying process.

Once the blemishes are filled, apply another Black Filler/UV Blocker coat and repeat the process. Don’t wipe the sanded part with surface cleaner before reapplying the next black coat unless the part has sat around for a while. If it has, just wipe down the surface with surface cleaner, but remember that you may remove any Wonder-Fil you’ve just applied if you do. Also, proper technique for surface cleaner is to use a lint-free shop towel or virgin cotton rag. Wipe the surface, turning the rag until the surface of the part is dry.

On RVs, composite parts may blend into metal parts. Here you see the White Filler/UV Blocker used on the sheet metal blending into the Black Filler/UV Blocker used on the composite parts.

Use only known surface cleaners; our preferences are on our product list. Enough problems arise when painting an aircraft without creating one with unknown chemicals.

One note on Wonder-Fil: We use it primarily to fill open pinholes. It will fill other areas, but the thick Filler/UV Blocker really works best. Previously filled pinholes may be visible, as the black blocker will soak down into the hole and sometimes leaves a slight depression. The next Filler/UV Blocker coat will level them out quickly. Wonder-Fil can be used at any step of painting—all the way through clear coat, as it turns whatever color you are spraying.

Repeat the priming process until you have a totally smooth, flat looking sanded part.

Many builders who use automotive paints normally choose flat looking gray primers, and the surfaces will look great until shiny paint or clears are added. I do not use typical base coat/clear coat. The initial priming steps never fully remove the blemishes, as flat gray finishes help hide them. Shiny sanded Filler/UV Blockers let you work blemishes out in the earliest stages of the painting process.

The details covered for the initial Black Filler/UV Blocker apply to all the rest of the painting process. The fundamentals are the same: air pressures, dry times and mixing ratios can vary, but overall after your first smooth composite part you’ll be on your way to being a painter!

Application of White Filler/UV Blocker

The smoothly sanded composite part can now have the final priming coat of White Filler/UV Blocker applied. Spray it on with a single-coat method or several lighter coats. The goal is to cover the black with white so that the color coats to follow will look top notch and vivid. Most bright colors jump out or pop when applied over a white base. If your parts (or your airplane) are going to be painted a dark color, you can simply use the Black Filler/UV Blocker under the dark final-color coat.

After sanding the white coat with 400-grit dry sandpaper, the part is ready for color. If you want to wet sand, that’s fine as you should have all areas sealed from water by now. A 320- to 400-grit paper is good for wet sanding.

A maroon or gray ScotchBrite pad is used right before color is applied to remove any shiny areas and to help assure proper paint adhesion. I suspect that when one sees clear coating peeling off of cars or planes, it’s because the surface was not scuffed prior to clear coat application. Even base coat that has been previously applied several days or weeks before generally will need sanding and even repainting before adding the clear coats.

Painting Metal Surfaces

Let’s discuss the application of our Filler/UV Blockers (primer/surfacers) on metal parts of a Van’s RV. Normally I apply only White Filler/UV Blocker, as we’re not trying to find composite blemishes or even block UV rays for this step, only apply primer. One may question why we want flexibility on the metal parts. All materials will expand and contract with heat, and metal can really expand. The Loehle Ultra-Flex technology is designed to delay the effects of the paint drying and becoming brittle.

Areas on RVs that blend composite parts to metal will have Black Filler/UV Blocker applied in those areas. The White Filler/UV Blocker coat on the composite will blend into the White Filler/UV Blocker used on the sheet metal. Remember, the black just helps you make glass areas completely smooth by showing you the blemishes.

Aluminum surfaces are prepped by a thorough scrub down with an alkaline detergent such as Dawn coupled with abrasive pads.

Aluminum Prep: Clean and Sand First

Before you apply any White or Black Filler/UV Blocker to the sheet metal, you will need to prep the aluminum. Entire Internet bandwidth has been used up discussing this subject over the years! Our methods uses two processes. The first is the industry standard of aluminum etching followed by Alodine. The second is the use of Pre-Kote. Both methods are great, but I prefer Pre-Kote because it seems easier and is environmentally friendly.

To begin, you need to thoroughly scrub the surface you are working on with an alkaline detergent (such as Dawn). Abrasive pads should be used with the detergent to assure the surface is completely clean. This will also scuff the surface and allow the next chemicals to work properly. Rinse with fresh water the whole time you are cleaning the surface. Do not allow the detergent to dry on the surface.

If you have rough areas, they can be sanded lightly with 220 to 400 grit sandpaper. Areas on an RV that require some of this are some slightly protruding rivet heads. Use common sense and be sure not to sand away excess thickness or even remove the Alclad (pure aluminum) layer on the metal. If water beads up on the surface of the metal, it’s not ready for metal treatment or paint.

On RVs, composite parts may blend into metal parts. Here you see the White Filler/UV Blocker used on the sheet metal blending into the Black Filler/UV Blocker used on the composite parts.

When doing the Pre-Kote etching step, be sure to keep the surface wet for 2 to 3 minutes followed by a clean-water rinse.

Aluminum Etching: Application

People complain about having to spend the time to properly treat aluminum. But compared to the process of building and/or painting an aircraft, the work is trivial, and the effort will be worth it in years to come.

After the entire surface is thoroughly scuffed and cleaned, it needs to have a chemical etching process applied to it. Aluminum Etch is the chemical to be used with Loehle Aero Coatings. The chemical is diluted with water before it is applied. For heavily oxidized areas, mix the Aluminum Etch with a 1:2 ratio, one part chemical and two parts water. For lightly oxidized and other surface areas, use the Aluminum Etch in a 1:4 ratio. Do not mix the solution in a metal container. Mix and store the chemical in a plastic container only. Also do not use the solution at temps below 60 F.

The Aluminum Etch is a phosphoric-acid base cleaner and brightener, so you need to protect any areas that won’t be treated with plastic (polyethylene) sheeting and masking tape. Any seams in the sheet metal that might have excess chemical solution wick into them and cannot be easily rinsed should also be taped up. Any solution that does wick must be rinsed off before it dries.

Apply the diluted solution with a brush, abrasive pad or sponge. Wear rubber gloves, protective clothing and eye protection. Keep the surface wet with the solution for 2 to 3 minutes and then rinse the solution off with clean water. If the solution is allowed to dry on the surface, reapply it and repeat the 2- to 3-minute period and rinse it off while it is still wet. Don’t try to work on too large of an area at one time, or you may accidentally allow the solution to dry.

The solution should not bead up on the surface, or the scuffing procedure was not done properly. It should provide a film over the entire surface.

Alodine Treatment

The Alodine treatment is similar to metal etching. We prefer to use Alodine that colors the aluminum a golden amber color, because this helps you know you have the areas properly treated.

The Alodine is applied full strength while the surface is still wet; right after the Aluminum Etch process is best. Apply the Alodine with a brush, abrasive pad or sponge. Keep the surface wet with the solution for at least 3 minutes or the time specified by the manufacturer, and then rinse with clean water. You should see a nice, uniform golden color. Blow everything dry with compressed air.

The structure is now ready for the application of White Filler/UV Blocker. Handle the surface as carefully as possible and try not to get oily fingerprints on treated surfaces. Applying the surface primer the same day is best. Pre-Kote works similarly, but it is a one-step process instead of a two-step.

The Color Top Coat will take longer to dry than the Filler/UV Blockers or Clear Top Coat. Best to keep the dust to a minimum during this process.

Applying Color

Sand the White Filler/UV Blocker lightly with 400 grit sandpaper and follow it with a maroon or gray ScotchBrite pad to remove any shiny areas. This light scuffing allows you to do a final inspection of areas before you go to the color stage. Any problems can be sanded and re-primed with white as needed. There is no reason to apply a new coat of white over the good surface areas. Simply lightly sand to blend the touch-up area into the other area, and you’re ready for color.

Loehle Color Top Coat is then applied. Standard colors are simply matches of the well-known colors that have been used in aviation for years. Special colors such as metallics are available.

I use an HVLP spray gun with a 1.4 to 1.5 size tip for spraying the top coats. The mixing ratio is 8:4:1:1 or eight parts color, four parts Universal Thinner, one part Color Top Coat Catalyst and one part Flexible Additive. Air supply is between 8 to 10 psi at the cap for HVLP guns and 45 to 60 psi for HVLP and siphon-feed guns.

Because modern urethane paints take longer to dry, they may creep under even the best paper tapes. Plastic tape is designed for this purpose and will pull off cleanly once the paint is dry.

The Black and White Filler/UV Blocker and our Clear Top Coats do not require the addition of Flexible Additive to them as it is already mixed in at the factory. The color, however, requires the flex to be added when it is time to spray. The reason is colors are mixed using gram scales and various toners, and the weight of the Flexible Additive would change all of the formulas and mixing ratios.

The spraying technique is different for applying the Color Top Coat. Colors are thinner than surface primers. They will run more easily, so we spray on a “tack coat” and allow it to tack up. This can be checked by touching an area that has been masked. This tack coat is essential when spraying metallics, as the flakes will move around if you don’t apply it. Also, a good tack coat will help prevent paint from bleeding under masking tape. This is especially helpful when you apply trim stripes or artwork.

Follow up the tack coat with a single heavier coat in the opposite direction. Allow this coat to flash off somewhat before applying a third coat. Some colors get by with a tack coat and a single full wet coat; others require more. Some metallics are best put on with many lighter coats. (See the Sidebar on Page 45 about how to clean your spray gun between applications.)

Mask any trim lines with a good quality plastic tape.

The Color Top Coat will take longer to dry compared to the Filler/UV Blockers and Clear Top Coats. Try to keep dust down as much as possible during this process. We generally apply colors and wait until the next day to sand or scuff them.

When applying additional trim colors to a previously painted surface, you will want the paint to be dry enough to scuff sand without leaving tape marks. Any tape marks that remain will disappear with a light sanding before the clear coating is applied.

Lightly sand the shine off of the Color Top Coat to get it well-prepped for the final Clear Top Coat. Wet or dry sand lightly and follow it up with a gray ScotchBrite pad. Go easy with wet sanding, especially on the very edges of sheet-metal joints. It’s possible to remove edges on RV wings when sanding them. A nylon pad works well for this.

The thickness of the Clear Top Coat means fewer coats to get good results.

If a part is accidentally sanded down to the White Filler/UV Blocker, use a touch-up airbrush to lightly blend in the Color Coat. This works well and the paint droplets are usually fine. Sometimes the brand of airbrush will require the use of more thinner and numerous coats to blend the color exactly, which is simple enough to do.

Modern urethane paints generally take longer to flash off or dry than older lacquers or base coats. They will seep under most paper tapes, even the best brands. The plastic tapes were designed for this purpose, and they normally pull off cleanly after the paint is dry.

Lightly sand the edges of trim stripes with 400 to 600 grit sandpaper to remove the slightly raised edges left by the masking tape. The thin light green type plastic tape is not as thick as the blue type, but one must be careful using it as it will stretch easily. When it stretches, it will narrow and mess up good, straight paint lines. The thin green is used for things like flames, but most airplane painting is best with the more durable, thicker blue tape. Once all is painted in color and scuff sanded, the final clear coat is next.

Clear Top Coat

The Loehle Clear Top Coat is formulated to be applied in as few coats as possible and to be flexible and thick. The top coat is also designed to flash quickly so that dirt and bugs will be less apt to be trapped.

This is the “wet” looking finish on a Lancair fuselage after it has been polished.

A painter can extend the time it takes to flash off with our coatings simply by going to a slower setting (warmer) thinner. If normal temperature calls for a T-3 (70 to 85) Universal Thinner, a T-4 or T-5 can be used. Some painters like to put on a good bit of clear and let the slower setting catalyst or thinners flow out the paint droplets to reduce orange peel. I find that a slight orange peel (like most factory painted cars) is better than runs!


Sand the Clear Top Coat first with 800 grit sandpaper. It also works well when dry sanded, but eventually small dust buildups form on the paper and will cause scratches. Wet sanding is better, and we are working on developing a sanding system that will machine sand using wet type paper so that sanding and polishing the whole aircraft will be really quick.

Sand first with 800 grit, then 1000, followed with 1200 grit. The final step in sanding is a wet sanding process on a 6-inch DA machine sander using 3000 grit paper. The clear will shine to a semi-gloss when the 3000 is used.

The machine polish step is done with a 7-inch buffer running about 2000 to 2500 rpm with a foam buffing pad. A special cutting compound is then applied with the buffer. The clear will actually slightly soften and turn to mirror-like glass. A good quality wax is applied after the paint cures.

Cost Savings

For years, I have heard builders say that they really don’t care how the plane looks as long as it is airworthy. The truth is that many of them would like a nice looking plane, but are either not sure how to do it or they want to save money at the end of their projects. If they take this approach, what they will see every time they walk into their hangar is a rough looking or dull finished airplane. And the truth is, if the finish is poor, people may also wonder about the rest of the workmanship.

For most pilots, a homebuilt airplane is a source of pride and joy. Why would you want to skip on the finish? Also, should you go to sell your plane, the shiny ones tend to sell faster. Enough said.

A completed paint job on a 5151 Mustang wing.

For more information, starter kits or quotes for materials, call Sandy Loehle at 931-857-3419 or visit www.loehle.com.


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