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Use the same methods as you would to repair automotive fiberglass. You'll find lots of information at your local body shop. Stay away from the use of silicones on anything you intend to paint. Be careful when sanding. Some fiberglas sousas were 'gel coated'. Once you are through the gel coat, you will be into raw fiberglass. Like I mentioned... solicit the help of an auto body man.

I used regular spray paint that you can buy from WalMart under $1 a can. I then used a clear spray to seal it and give it a shine. I painted them black so no priming was really necessary. I just painted over the old finish after washing it. I admit that I hate the sound of a fiberglass sousa but you can have fun decorating them for parades and such. We had black sousaphones with cobwebs hanging from the bells for a "nightmare" themed marching show. It had a cool effect.

TubaTwo wrote:What should we use to paint it?
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What should we use for filling cracks?

I would use fiberglass cloth (trimmed to cover the crack) dipped in fiberglass resin (to which a few drops of hardener have been added). Get these materials at an automotive or hardware store.

Sand the fiberglass/resin repair area smooth after it has hardened.

Read this or a similar book at your library:
Fiberglass Repair and Construction Handbook

No need to prime before painting, but wash off the fiberglass parts with lacquer cleaner and a clean, lint-free cloth, after you have removed every last trace of silicone.

Hang up to paint in a clean, dust and insect-free area. Spray paint with an epoxy, white-colored "appliance"-type paint from the hardware store.