I was looking at the composite tanks for my O'day 35, but was concerned about fitting in the locker, and the price. The bottom of my steel 10lb tank looked pretty bad, and it was 16 years old.
My old tank looked like the one on the left in this picture (except with a lot more rust around the bottom)
I bought a new 10lb tank for ~$60 (including shipping) filled it ($4), then coated the bottom "ring" with 4200 UV Fast Cure in white. The 4200 UV matches the paint perfectly. It looks like it came that way, and I believe that it won't ever rust. The new tank looks like the one on the right.