Once you’ve done all of the metalworking, it’s time to sand and polish your frame. There might be little lumps of filler metal from brazing, or stray file marks, or lumps in the castings of parts you’re using, or inconsistencies in the welding, etc. All of these imperfections will show up through paint. If you care about those aesthetic considerations, any of those blemishes need to be blended out with sandpaper.
I like 1 inch wide rolls of sandpaper in varying grits. They are nice because you can run them over the tubes in a shoe polishing motion, or wrap them around a file. Be careful not to remove too much material by sanding. They smooth out the surface of the metal tubes by removing metal and it’s totally possible to sand right through the tubes. Don’t create a thinner tube wall than you designed for by sanding too much.
This is particularly easy to do if you’re using sandpaper to smooth out fillet brazed joints.
There are a couple options for coating your frame:
Powder Coating uses electrically charged plastic particles to coat the metal frame, and then melts them all together in an oven. Powder coat is more durable than wet paint, better for the environment, and almost as good looking. It’s also much less expensive and much less labor intensive. It’s a common industrial process and most medium size towns will have a local powder coater. To get an idea of what colors are available, call your local powder coater. You could also talk them into doing other colors.
Automotive Wet Paint.
Automotive wet paint is also an option. Talk to your local auto body shop and see what they can do. Unless you want to seriously nerd out on painting, it’s very unlikely that you can do as nice a job as an automotive body shop at the same cost.
I have wet painted some motorcycle and scooter stuff, along with a few bike frames. It requires control of the humidity in the atmosphere, scrupulous attention to detail, fastidious cleanliness, and a ton of sanding. Getting good at wet paint makes climbing the TIG welding learning curve look easy.
This is the stuff you buy at the hardware store. Follow the instructions, use primer and a top clearcoat. It won’t be very durable, but you can do it in your garage. Emptying a spray can releases lots of stuff you don’t want to breathe. Do it in a well ventilated area. I spray painted a lot of things in my parents’ basement when I was in high school. It stunk up the whole house. I can’t believe they put up with it.
I haven’t used it, but it’s in rattle cans and marketed for coating bike frames.
Direct to Metal (DTM) Epoxy Paints
These are really durable coatings that you can apply with a brush. I rode two bikes painted with this stuff and while they weren’t pretty coatings, they worked great.
If you want to go for a post apocalyptic form follows function look, coal tar epoxy is a good option. It’s ugly, goes on with a brush, leaves brush marks, and doesn’t require primer.