Flying for Fun
D.B. Mathews 909 North Maize Road, Townhouse 734, Wichita, KS 67212
And Furthermore
Last month I took a quick look at model aircraft finishing systems that lose part of their contents to reach a final form; they change from a liquid to a solid state by evaporating solvents. They dry, not set.
In that discussion I mentioned the use of nitrate and butyrate dopes on paper- or silk-covered models. A superb video by Larry Kruse that demonstrates lightweight covering and finishing techniques for models is available from Robin's View Productions (Box 68, Stockertown, PA 18083). To paraphrase the old Tonight Show, "everything you could possibly want to know is contained in this sealed videocassette."
This month I'll overview some materials that depend not on losing chemicals, but on adding two or more chemical compounds together to create a different material. This phenomenon is referred to as an "addition reaction."
Catalyzed materials are commonly referred to as "epoxies," but they can be polyurethanes (Imron, for example), enamels (Centari), or true epoxies (Hobbypoxy). Even the "modeler's friend," cyanoacrylate adhesives, are catalyzed by moisture and a change in pH from acid to alkaline (that's what "kickers" do). Other compounds are catalyzed by heat, pressure, or light.
The finishing materials commonly used in modeling are created by addition reactions. Adding Part A to Part B or a few drops of catalyst starts a curing action; they are catalyzed by addition only.
The field of catalyzed chemical reactions is incredible and far beyond the scope of my knowledge. Catalyzed reactions are at the heart of the series of industrial processes used in refineries and chemical plants to convert petroleum into everything from clothing to pharmaceuticals.
Hobbypoxy
Hobbypoxy is an epoxy finishing system introduced to the model airplane hobby in March 1963 and has remained a favorite ever since. Pettit Paint Co. of Belleville, New Jersey had previously been successful in the marine finish field. Bev Smith, a member of Pettit's marketing staff and a modeler, influenced the firm to market the product for modelers' use.
Remember that this was in an era when really pretty finishes were 20 coats of hand-rubbed dope. Only the most diligent and expert modelers were capable of such shiny finishes, and most were seen on competition Control Line Stunt models. Others stood in awe of those modelers' craftsmanship. Then, with the introduction of catalyzed epoxy finishes, even we duffers could create finishes comparable to those dope beauties — but simply and quickly.
The material was loaded with pigment when compared to the thinness of butyrate dope, because dope has to degas from the bottom out and thick coats invariably trap solvents, leading to all sorts of misery as they "dry." On the other hand, epoxies "cure" internally, releasing very little volatile material; therefore they can be much richer and thicker in color coats.
There was a learning curve with these new epoxies, and some modelers had difficulty. The material could be brushed, but really needed to be sprayed to reach its maximum beauty, and the spray guns and compressors available were large, expensive, and well out of the domain of hobby items. Gradually, less-expensive, less-maintenance-sensitive spray equipment was developed for hobby use, and modelers learned the required techniques.
Modelers also quickly learned to heed the manufacturers' advice about mixing products in successive coats. Hobbypoxy's fillers were meant to be used as base coats and were superior to "sanding sealer" dope. Equally impressive are the much harder surfaces compared to dope. Epoxies are infinitely more durable, they set in less than a day, and they even resist 60% nitro fuel spills.
The catalyzed epoxies represent a huge leap forward in modeling technologies and make it possible for us marginal craftsmen to produce previously impossible finishes. I've included a photo of George Sauer's big five-cylinder O.S. radial-powered WACO as an example of a superb Hobbypoxy-finished model. George has a reputation in this area for superbly done airplanes and uses only Super Coverite and Hobbypoxy. Although some of his models are rather old and much-flown, they still look splendid.
Hobbypoxy is still available 35 years later, with no discernible change in chemistry, though the can labels have been modernized. Good stuff then, good stuff now!
Super Poxy
K&B introduced its catalyzed finishing system shortly after Hobbypoxy was introduced. Small technique differences existed, such as eliminating the need for 45-minute waits after mixing, more rapid setting times, and variance in flow rate.
Any discussion of Super Poxy's merits is superfluous, since it has been forced off the market by the EPA (Environmental Protection Agency).
Ultrapoxy
This is being written in late September. Less than a week ago I received samples of K&B's replacement for Super Poxy and hurriedly did some nonscientific testing. The following is personal opinion: for once, a new and improved product actually is!
At the risk of upsetting someone who has a far better understanding of organic chemistry than I, it seems that Ultrapoxy Part A, which contains toluene and ester solvents, combines with Part B's polyamides and butyl alcohol to create a polyamide ester and a much safer alcohol derivative of toluene.
Mixed or unmixed, this and any other catalyzed finishing materials have health risks if handled carelessly. This magazine's "Safety Comes First" column has reported a number of serious health problems associated with the misuse of this group of finishes. Simply put: don't breathe, swallow, or dab yourself with these materials. Filter masks are easily purchased; lungs are not.
One should also wear a head cover, such as a cap, when painting with these materials. The reason? One's scalp is a very porous piece of skin, making passage through it much easier than through, say, one's hands. Not only that, but if the painter still has hair, it holds the overspray in contact with the scalp for a long time.
Color Matching
As I write this, Ultrapoxy is available in:
- White
- Bright Red
- Black
- Medium Blue
- Yellow
- Cub Yellow
- Orange
As you may recall from your crayon days in elementary school, yellow, red, and blue are the three basic colors from which all other colors can be made. Black and white change the chroma or depth of the basic colors.
Since the most common use for catalyzed finishing materials is cowls and pants on models covered with one of the heat-shrink Mylar™ films, how does one match the colors? K&B will have a color-mixing chart for this purpose. Theoretically, it should be possible to mix and match any covering material's color and chroma, except for the metallic shades.
One needs accurate measuring devices, such as disposable plastic pipettes and graduated cylinders, to measure the specified quantities of the basic colors to match the heat shrink. Dealers might love this, since they will not need to stock a zillion premixed matching colors, and neither will we builders!
So How Does It Work Already?
Although Ultrapoxy is thin enough to spray without adding thinner, it is remarkably rich in pigments and is very smooth. Flow is excellent; a light tack coat followed by a flow coat about 15 minutes later produced no runs (as few as I've ever encountered with any spray finish). The material can be brushed fairly well, with minimum brush marks, but spraying is vastly preferred. Next month I'll report on an economical spray gun to resolve this nagging problem.
Panel A in the photos was covered with clear Worldtex, followed by two coats of clear nitrate dope. On top of that I brushed two coats of white Ultrapoxy, then allowed everything to cure for 24 hours.
To assess compatibility with other finishes, I then masked the panel with the 3M™ tape mentioned in last month's column and brushed on the following from left to right:
- Rust-Oleum™
- LusterKote
- Krylon
- 21st Century Paint
- Perfect brand polyurethane
- Super Poxy
- Hobbypoxy
- Butyrate dope
- Nitrate dope
- Super Poxy primer
All finishing materials tested stuck to Ultrapoxy with no bubbling, crazing, or lifting. I have never seen a material this compatible.
The reason why some of the samples on Panel A look smeared is that all were rubbed with a rag soaked in 10% nitro fuel. LusterKote, Krylon, butyrate, and nitrate dope did not stay put when subjected to rub fuel.
Panel B was also covered with Worldtex, given two coats of nitrate dope, masked, then painted with the same group of finishing materials from right to left. All adhered well to the clear nitrate dope. After curing, I masked a strip and brushed Ultrapoxy on top of them. Again, to my surprise, Ultrapoxy covered the darker colors and was compatible with all brands tested. After it had cured for a day, I rubbed the dickens out of it, and no flaking or scratching was evident.
The Verdict
Although putting Ultrapoxy underneath or on top of any of the listed finishing materials is neither particularly wise nor necessary, the fact that this new material is of such low volatility that it makes the preceding tests successful is very interesting to me. The resistance to nitromethane and hardness of the cured surface are exemplary. And perhaps most important, Ultrapoxy is user-friendly. It's as good as the rest and better than most.
More To Come
Next month I'll look at a catalyzed automobile enamel and an inexpensive device that makes it possible to spray additional reaction materials from the equivalent of a spray can. In the meantime, hopefully I've stimulated you to investigate some finishing materials that can increase your "Flying for Fun."
Transcribed from original scans by AI. Minor OCR errors may remain.
