As a general rule, the harder the surface is the more mirror smooth it can be polished to. So if you want that mirror smooth reflective surface, you probably want an epoxy finish. Of course that hard surface will be difficult to rub and buff. You will probably need a metal polish and powered buffing wheel. On the other hand, dope is relatively soft and you can rub and polish the surface smooth by hand with a common rubbing compound and an old T-shirt. Wax is not really a part of finishing, but it helps protect the paint from oxidization. Wax can sometimes restore the luster of a blushed paint job. If you have sealed the wood, but not filled the grain and painted it with a dark color, wax can partially fill the deep wood grain giving the appearance of white veins. This may even reduce drag, at the expense of calling attention to your poor finishing.
In my rushed contest building, I often do not have time (often due to weather) to spray the color coat on my rockets. I take them to the contest and paint then as soon as I arrive at the contest field. I'll hit them with a spray can of red or orange Pactra or Sig dope. Unfortunately, I cant find spray dope anymore. I don't want to drag out an airbrush, and then have to clean the airbrush. The dope dries fast in the hot summer sun, I rub down the surface with rubbing compound, and I'm ready to fly in short order. You can't do that with an epoxy finish.
There are two schools of thought on epoxy finishing that I will not be addressing, HPR composite construction, and ultra light FAI models. You can find discussion of those elsewhere. Rather, I am a fan of the old Hobbypoxy and K&B products.
I have used Hobbypoxy Formula II for finishing. Formula II was a thickish 50-50 mix epoxy that had lot of uses. I used it on the balsa fins of a k-36 Saturn V. This gave me a hard durable finish with sharp edges. I have also used it for finishing molds for vacuum forming, I would turn hard oak on a lathe, turning it a bit below my final intended surface, and then slather on a layer of Hobbypoxy Formula II. You do have to keep rotating the part until it sets up, so that it does not flow or slump. However, the real secret is that before you can do final machining in the wood lathe, you need to heat cure it. Put it in an oven on warm (140-160 deg.) for a couple hours. So epoxy finishing works great for some applications. Hobbypoxy is long gone, but I'm sure another suitable epoxy could be found. I also liked the HobbyPoxy Stuff filler.
HobbyPoxy and K&B both had two part epoxy paint that was sprayable. The thinner would dry off quick enough, but the epoxy paint itself takes a while to set up and cure. During this time it can collect dust etc.; a heat lamp can help speed up the cure. I used the K&B paint on one test rocket. Meh. I also used it on a sailplane fuselage where I thought the durability would help on landings. The product that I truly loved was the K&B epoxy primer.
more later
Sorry for the long delay.
The The K&B Primer, now Klasskote, is awesome for finishing tubes, and works well as a primer/filler on other surfaces as well. You can brush it or spray it with no surface prep, it sands easily and never loads the sandpaper. There is never any tendency to flake or separate and it can be painted with any type of paint. You do have the inconvenience of having to mix a small pot for each use. I usually use up the rest of the pot by painting additional tubes.
I also tried a "one step finishing resin". It turned out that the product was just a polyester fiberglass resin. My first attempt resulted in some over sanding of the hard material. With a little practice it could be done in one step. However, there is still the heavy smelly yuck factor. Don't be afraid to try new things, or even old things with fresh packaging and marketing.
Balsa is very soft and the first thing you want to do is lay down a hard base layer. This could be a coat of sanding sealer, clear dope, random paint, CA or Epoxy.
For training purposes, this layer could be colored. After sanding off the fuzz, the filling begins. Ideally the filler should be an easily sandable material. My preference is Clear dope, or sanding sealer, followed by a dope based filler coat. Now the sanding sealer is just clear dope with small amount of talc added. The balsa filler coat is thinned dope with lots of talc added. Typically you can add even more talc. However, the filler coat will be a bit porous, so I top it off with final coat of sanding sealer. Otherwise, when you spray on that thin layer of color dope you will not get a smooth finish. Talc has a bad rep these days, but talc is just powdered rock, MIca , a flaky mineral, and is harmless. However, geologically, Asbestos, a hazardous fibrous rock mineral is often found near mica deposits. Some miners were negligent and put Asbestos into the talc product stream. If you don't trust talc, you can add just about any other powdered solid. Estes used to sell Astroseal with powdered balsa filler.
Sanding is one skill you need to get that perfect finish, You want to sand down each layer of filler just to the start of the slightly harder base layer. If you sand through the base layer, you essentially have start over on that patch. After each coat, inspect to see of the grain and other defects are completely filled. If not, apply another layer. After some practice, you will know when to stop sanding, and even how many layers of filler will you need. Eventually your skill will improve to the point that you do not need to sand between layers, just sand off any brush hair, runs, bugs, etc. between coats, then sand once. The choice of sandpaper is up to you. I like 3M wet or dry paper, but I only use it dry on models. The dope products will accumulate or load up the sandpaper. You have to watch out for this and change to a clean sheet as needed. Loaded sandpaper can often be cleaned by scrubbing it under running water with a stiff tooth brush. Loaded sandpaper can but deep groves in the finish, and it is also possible to gouge the surface with the edge of the sandpaper, as well has just over sanding through the base layer. The real secret to getting that perfect finish is not the materials used or even your skill. It is recognizing that finishing is a discipline. If you make a mistake, you just to keep at it until it is perfect.
Perfection is not all that it is cracked up to be. The Persian flaw is a real thing. I remember putting a perfect finish on an Estes Starship Vega, with die crushed fins. I proudly showed it to my uncle who got me started in control line models. He was not impressed because he thought it was just a plastic RTF model.
I could go on endlessly, but I've said enough, spread over 3+ posts. You should also read finishing articles from other sources, like model airplanes, Plastic model builders, and professional model builders.
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