TubaForum.net

My King fiberglass sousaphone has a few cracks at the bell rim. They range from 1" to 2". What it the best way to prevent them from spreading?...

Thanks.

Drilling a tiny hole at the base of the crack should stop the crack from spreading. It is fairly easy to patch cracks in fiberglass with glass mat and epoxy resin, but of course it will show. You could try epoxy putty. I think it comes in white. Don’t know if there’s a white version of something like JB Weld but I think that could work too.

The round hole thing is probably fine, but (having tried all sorts of glues to address this problem, as others went beyond your question to discuss adhesives) I would probably use PVC cement.
It has proven the most successful (for me...as it seems to melt the plastic back together, rather than just "glue" it), but (as two thin edges of old brittle plastic are being glued together) I don't believe there is any cement that will prove to be as strong as "new/never-cracked".

Unlike plumbing joints, I believe I would let the glued bell set for at least a couple of days.