Gluing and Painting Plastic Parts

As seen in the October 2022 issue of Model Aviation.

FLYING SEASON IN THE high desert is in full swing, but we’re still windier than usual, so while I’ve been waiting it out, my workbench has been quite active.

One thing that is ongoing is not only cutting balsa but also building and painting plastic parts and accessories. In that vein, questions came up from Mark Mamisano not only about what adhesives work best for vacuum-formed styrene and polyethylene terephthalate (PET) plastics, but also what types of paint work best for the different types of plastics. They are good questions, so I’ll touch on what I’ve learned throughout the years on both counts, as well as effective ways to prepare the plastic for paint.

Regarding the type of adhesives that can be used, I’ve been dabbling in plastic modeling for more years than I’m willing to admit, and I have found that there’s a good bit of crossover between plastic modeling and assembling vacuum-formed parts for RC models. The main thing is selecting the right glue and paint for the plastics that are used.

These are the basic adhesives and weld cements that are used to assemble vacuum-formed plastic cowlings, wheel pants, and other accessories that are provided in many of today’s RC model kits.

For styrene parts, you can get away with using medium gap-filling CA adhesive, but don’t use accelerator because it causes the plastic to become extremely brittle—in some cases, so much so that the plastic will actually crumble under even the slightest loads. Thin CA also crystallizes the plastic, so avoid that altogether.

I have two preferences for styrene. The first is Squadron Weld cement, which is a thin, clear liquid that doesn’t actually "glue" the surfaces together but gently melts the surface of the plastic parts so that they fuse together. It’s applied with a paintbrush, where capillary action wicks the cement from the brush into the seams. When the liquid evaporates and the plastic re-hardens, the parts are welded together.

The second choices are Faller Expert and Faller Super-Expert adhesives. These are truly "glues" that create a chemical bond between the pieces. I prefer Super-Expert because it sets up faster than the original Expert does, although the full curing time for both is roughly the same.

I’ve also used a nontoxic (water-based) Micro-Weld liquid weld cement, but it slowly softens the plastic, making it far more difficult to keep things aligned unless they’re clamped together, which is sometimes difficult. In the end, the bond is good; it’s just excruciatingly slow to work with.

Getting into the white PET and clear polyethylene terephthalate glycol (PETG) plastics, medium CA works great, but it bonds quickly, so the parts must be aligned carefully to avoid misalignment from the glue taking hold too quickly. I’ve also used Faller Expert with excellent results. For filling the seams, catalyzed Bondo Body Filler works great, with Bondo Glazing & Spot Putty to fill the slight imperfections.

Shown are a few of the lacquer, acrylic, and enamel paints used to paint vacuum-formed parts. Other brands work equally well, as long as they’re compatible with the type of plastic with which they’re used.

Where painting is concerned, you need to stick with enamels or acrylics for styrene. Lacquer-based paints or dope will, at best, etch the plastic, making for a rough "orange peel" look on the surface, or, at worst, melt the plastic and distort the entire part.

To prepare for painting, I wet-sand the entire surface with 600-grit sandpaper then wash it down with Dawn dishwashing liquid to remove any sanding residue or leftover oily residue from the molding or handling process. Before the color goes on, I prime the surface with a light gray primer that provides a more opaque finish. Doing it that way provides better, more even coverage and uses less paint overall.

I’ve also used Alclad II Lacquer natural metal finishes for unpainted "aluminum" surfaces, but it only works well on PET and PETG plastics. Experimenting with different types of paint and glue is also encouraged, but it’s a good idea to experiment with scrap plastic before you commit to the parts that will be used on your models to avoid any major setbacks or surprises.

One other question that came up this time around was about good sources for Polyspan. The best place I’ve found is Fly2Build. It’s available in 10-foot × 38-inch or 20-foot × 38-inch bolts. I went with the 20-foot bolt the last time because you can get more coverage with less scrap than with the smaller bolts. I can tell you firsthand that the service from Fly2Build is excellent.

Reader Submissions

Joseph Lemieux built this Cessna Bobcat from the Dare/Brodak Songbird kit. The 30-ounce model is a very stable and docile flier and would make a great "first twin."

Let’s get into what the readers are doing. First up is the Cessna Bobcat from Joseph Lemieux.

Joseph built his model from the 48-inch wingspan Dare Songbird kit by Brodak Manufacturing. The model is set up using brushless motors with 9 × 6 counter-rotating propellers and 9-gram servos. It sports yellow iron-on covering with hand-cut black trim. The Bobcat tips the scales at 30 ounces with a 2,000 mAh 2S LiPo battery, or 32 ounces with an 1,800 3S battery.

The maiden flight was made using a 2S battery. Joseph reported that it flies beautifully, cruising at approximately 3/4 throttle.

Michael Myers’ Morane-Saulnier L is from a kit available from Brodak Manufacturing and was built with electric power and four-channel RC. The Morane is set up with wing warping for lateral control, as was its full-scale counterpart.

Last but not least, Michael Myers is at it again—or should I say still? This time around, he took a step back in time and built the Andrew Hewitt Free Flight MoraneSaulnier L from Brodak Manufacturing.

The kit was updated recently for four-channel RC and set up with wing warping using a single servo to actuate the bellcrank that twists the wing. The model is covered with Polyspan, which raised the question as to whether the wing would be too stiff to twist.

In the end, it was not, and watching the wing twist gives some idea of what the pilots of those old airplanes experienced back in the day. The good news, though, is that despite the primitive methods, the system works fine, and the Moraine is a terrific-flying model.

With that, I’m going to have to put this up for now. I hope the section about gluing and painting plastic parts will answer a few unasked questions and make building up plastic parts easier and more successful. Thank you, Joseph and Michael, for sharing your latest endeavors. Both are beautifully done.

Until next time, do keep the good stuff coming. There’s nothing I like better than seeing a beautiful model that was built with your own hands.