Sorry, but paint just doesn't belong on an acrylic hot tub. I don't know a lot about fiberglass repair, but my guess is that the best option would be to contact your local hot tub repair and service businesses in your area.
One website I thought was more helpful than most is: http://www.poolspahelp.com/spa_shell_repair.htm
It only gives ideas about repairing cracks, but it sounds like your finish has oxidized, and needs a new coat of acrylic.
best of luck to you, just stay away with that paintbrush!