Control Line
Aerobatics
Wynn Paul
Picking up where I left off last month talking about painting my latest creation, here's what Windy Urtnowski has to say about repainting old airplanes with Imron.
Windy: "I've used Imron over silkspan that has old dope on it, as the silkspan and dope seem to stay flexible with age. As an experiment I refinished a three-year-old Ringmaster: I cleaned it well, wet-sanded with No. 600 paper using Prep-Sol, toweled it off and sprayed clear Imron over the old dulled-out finish. It worked great, and my Ringmaster helped me get first in OTS at the 1982 Nats! Lou Dudka has done a similar experiment with an old Matrix and it came out looking brand new. This process is definitely worth considering if you have an old ship that flies well, needs nose weight removed, and can stand the extra ounce of weight on the wing loading. The weight will be mostly behind the center of gravity, so don't do it to a plane that needs two ounces of nose weight now."
On sanding and rubbing out Imron: "Runs in Imron can be block-sanded out using 400–600 paper, wet or dry, and the finish is very glossy without rubbing; however, rubbing is necessary for those final rows at the Nats where we all want our 'Pro-Stunt Ships' to shine during appearance judging. Glen Meador and I rub out planes with automotive buffers and electric drills, but many areas are still too delicate and must be done by hand. A dope finish or Imron finish will buff up pretty much the same after they cure for a month or so."
A reader asked, "How do you cure a plane for a month before the Nats when you finish it on July 20th?" Windy’s answer: put it in the sun every chance you get, both ways—upside down half the time—on a blanket.
Rubbing-out technique: "The trick is to wet-sand with No. 600 until the entire plane is dull, then buff as much as you can with a drill or automotive slow-speed buffer and a 3-M sheepskin (or similar) pad. There are still many areas that must be done by hand. You can make small pads from cut-down five-inch rubber drill pads (about 2-1/2-inch diameter), wrap small homemade sheepskins over them, and use a foot speed control to work small areas."
Compounds and polishes Windy mentions that work:
- Red rubbing compound (DuPont No. 0861N)
- White polishing compound (DuPont No. 0761N — finer grit than red)
- Silver polish (Haggardy silver polish: pink is smooth, brown is rough)
- Turtle Wax No. T-232 liquid rubbing compound
- DuPont No. 7
- Toothpaste
- Classic Car wax
"Anything with abrasives will tend to smooth down microscopic high spots and orange peel and thereby increase reflected light in one direction (which we commonly call shine). My personal setup is a quarter-inch drill with a foot speed control (sold by Sears) and Haggardy silver polish."
A few notes on Imron and similar two-part paints:
- Imron colors, like clear Imron, will go over any coating that is prepared to accept lacquer finishes (surface should be roughed up and clean).
- Imron clear is mixed four parts clear to one part hardener; tack time is about two hours.
- Ditzler clearcoat is mixed eight parts clear to one part hardener; Bob Baron used it successfully on his 1980 ship.
- These other two-part clears dry faster and are slightly less toxic than Imron, but they seem slightly less "bulletproof" and may lose shine faster. They can offer a small weight saving and pick up less dust due to faster tack time. Use a charcoal respirator when spraying any multi‑part paints.
- Imron seems to work well on structures that flex, such as I-beam wings and stabs.
Paint compatibility guide
- Imron will go over almost anything (provided the surface is roughed up), but dope (nitrate or butyrate) will not go over Imron.
- Sig dope (a butyrate type) will go over nitrate dope (like Aero Gloss). It can be used over or under acrylic lacquers. You can use most lacquer thinners with Sig dope.
- Aero Gloss dope (a nitrate type) will not cover a butyrate (Sig) dope. It can be used on bare wood without problems; it cannot be used over a two-part paint like Hobbypoxy. You cannot use regular lacquer thinner with Aero Gloss.
- Hobbypoxy will usually go over anything; use the Hobbypoxy thinner. You can use butyrate dope over Hobbypoxy if it is applied "dry" (a thick mixture without excessive thinner).
- Acrylic lacquers are from the same family as Sig or Randolph dope; they can be used over, under, or mixed with dope and use the same thinners. Treat them like butyrate dope and you'll be satisfied.
- Both Ditzler and DuPont make full lines of acrylic lacquer products, but they tend to dry out and become brittle with age. I would not recommend them on I-beam wings/tails or silkspan-covered surfaces.
Additional dope notes:
- Nitrate and butyrate dopes come in both high-shrink (tautening) and low-shrink (non-tautening, aka Litecoat) formulations.
- Nitrate dope is basically for use over bare wood or to attach covering to airframes. Nitrate-type dope will adhere best to bare wood or to fillets such as Expoxolite.
Many thanks again to Windy Urtnowski for his comments on painting. We'll continue with more on the subject after the Nationals issue.
Transcribed from original scans by AI. Minor OCR errors may remain.
