That was very interesting and very informative. You have some unique answers to common problems.
Since I used to be a partner in a body shop I would chose to use other methods of repairing tubes. The exhaust pipe expander it a good idea for small tubes. Once a larger tube is distressed, though, it is more difficult. I have a 6" f/15 refractor that was on loan to a friend while I was in Iraq. A violent wind storm came up and knocked the entire thing over. The tube has some pretty nasty dents. They will be removed by the old "hammer and dolly" method. This is where I take a bodyman's dolly and various hammers and work the metal back into the proper shape. Once you've done this to a lot of fenders and doors on cars it gets to be pretty easy.
Eventually, I will have the tube so close to perfect again that it will be impossible to find the repair areas. Of course, any very minor imperfections will be fixed with spot putty and not epoxy. If any areas need that much material then the repair just isn't done well. My partner and mentor came from the old school where if you had to fill it you had to go back and do it right! I should point out that this is not a criticism of your workmanship. It's just that when we built show cars that cost thousands of dollars we were much more particular about such things. That training becomes a habit over time. After all of the metal work is done, a good coat of Zinc-Chromate primer will be laid down using an air powered spray gun. Zinc-Chromate is superior in its ability to bond to aluminum when compared to regular primer.
The finish is only as good as the preparation. It is not normally necessary for rougher grades of sanding material. On some automobiles, where custom made curves or shapes are built with body filler, or fiberglass, a 60 or 100 grit would be needed. With telescope tubes I don't think I've ever used anything more coarse than 320 grit. Actually, I start with 400 or even 600 grit wet and dry. I use a sanding block at all times. Using one's fingers can lead to an uneven finish since the skin on your hands and fingers is pliable and will mold into small imperfections.
Normally, 600 grit is about the finest that one should use when preparing for primer. The secret, as I mentioned, is in the preparation. Laying down primer, as well as top coats, should be done with a nice fine coat. It is not a good idea to try and cover everything in one session. Lay down a mist coat and let it dry for about 5 minutes. Then another and if necessary, another, then a final coat. Once the primer has dried for about an hour (The temperature has a lot to do with drying time.) you can wet sand the tube. This is where patience and quality start to show. Use a garden hose or a bucket of water to soak the wet and dry paper. Again 600 is a very good choice. Sand the primer gently and keep wetting the paper. The water will carry away the small particles of primer that are being removed by the sanding medium. When the primer begins to show through ever so slightly it is time to stop sanding. Wash the tube and let it dry. Repeat this entire process several times until the primer is built up to a point where none of the tube or earlier paint show through. Let everything dry for as long as necessary.
Top coats are an interesting item. With the huge steps forward in enamel paint it is quite possible to get a very good coat with one painting session. Lacquers on the other hand, are a very time consuming process as well as expensive. That type of paint is best left to someone with a lot of experience in a body shop, so, I will not cover it.
Enamel can be laid down just as was the primer. However, there is no wet sanding involved between coats. Each coat should be done within about 10 - 15 minutes of the last. Some paints will allow up to an hour between coats, but, that just leaves room for a distraction to take you away from the work. After that happens you may need to wait as much as a week before you can continue!
Build up each coat in thin layers. Remember to start the spray off of the end of the tube, carry it across the entire tube and only after the spray fan has left the tube should you stop spraying. Repeat as often as necessary. If light coats are done with care and allowed to partially dry for about 10 - 15 minutes each, there is virtually no chance to have any runs developed. Any run in the paint means that you laid it down too heavily on the last coat.
If you need to have an special color, a trip to a body shop supply house is the best bet. They can mix and match colors that don't even show up in the chip books. By the way, do not be afraid to look through some of the very old chip books from DuPont and Ditzler. Did anyone realize that the paint used by Astro-Physics is the exact one used on the 1986 Corvette? Yep! Check the chip charts against your A-P and see for yourself!
As mentioned by Barry, paint does age. It is exposed to the sun, oxygen, moisture, pollution and all kinds of things that can change the color. If an exact match is needed for a repair job then finding the closest match is mandatory. Once it is close then the paint technicians at the supply house can use their expertise to adjust the mix until it is a dead on match. It may take some time, but, it can be done. I did exactly this on an old telescope. It took us several days, because of drying time for the paint, but, we eventually wound up with an exact match. There was no way to tell which was the original paint and what had been repaired. Oh, yes, paint DOES change its tone when it dries. That is one of the things that the paint technician should consider when mixing the paint.
I am one of those people that like to have some spice in astronomy. I've painted OTAs some very interesting colors. Barry, you weren't the first one with a purple OTA! I did that to my 1965 Jaegers OTA about 7 years ago! I have also painted an RV-6 in a nice blue metallic and an Edmund 4-1/4" in an Emerald Green Metallic! (I think that green was left over paint from my 1972 Dodge Polara!) Heck! I even painted a Celestron 80 with Kameleon Paint! That stuff is expensive! It is about $375.00 PER PINT and that does not include the thinner or hardener!! I will attach some pictures for everyone to examine.
The next thing is finishing the touches. With modern enamels a clear top coat is easy. The same techniques need to be used to avoid getting runs in the finish, but, once it has dried the color coat is safe for a long time. In the days of lacquer paint it was necessary to paint and wet sand as many as 20 color coats then do the same with the clear coat! It was a big pain and a lot of work. However, until enamel paints matured, that was the only way to get that deep lustrous finish that actually had a sense of depth to it visually.
Barry mentioned that enamel paints can take a longer time to dry than is apparent. It's not actually a drying time as much as it is a curing time. The paint surface can dry quickly. However, the thinners that are in the paint below the surface need to leach out and that takes time. Heat, as he mentioned, is a great way to accelerate the process. It used to be, and in some cases still is true, that if your car needed to have paint work done, you would be advised to not wax it for 90 days! Also, the only way to wash it was with cold running water and nothing more for that time period. That is because the paint needed to cure. Now, most shops have huge baking ovens to solve this problem. Still, unless you can put the entire tube, or whatever, into an oven and let is sit for up to six hours, it will need to cure. Again, temperature and humidity are an issue here. Since I haven't a large enough oven for big tubes, I hang them in a corner where the won't be disturbed and leave them alone for several weeks, even months, before I put them into service. This is the only way that I have been able to get the hard finish that I wanted and that wouldn't take an impression from the felt in tube rings when they were tightened down. I am now working on building a large and long baking oven just for OTAs such as the one for my refractor.
One last little thing that many people over look is trim. It's done with cars and boats. Why not with a telescope tube? I've done some nice simple lettering and pin striping on a few tubes. It was just basic stuff and was done with special tape because I have just never been able to paint straight lines by hand. Again, check the pictures and you will see some of that gold trim.
Barry, I almost forgot to mention something. Try the Rustoleum Appliance White spray bomb for the Unitron. I think that it will be about the closest match you will find. I'm pretty certain that the finish on the old Unitrons was the same stuff that was used on washing machines and dryers! LOL
OK! I've just about written a book! What can I say? This caught my attention and it is something that I do and do well. I guess it kind of takes me back to the days when cars were cool and unique and we hadn't a care in the world!! LOL
Clear skies to all and thanks for letting me carry on a bit.