Fiberglass tubs has turned yellow Can it be cleaned and with what?

Assuming it has a white gelcoat finish that has oxidised,clean it with acetone and clean rags, then cut it back with 1200 grit wet&dry paper with water,only until it turns white and smooth, taking care not to rub through it. It can then be polished to a high gloss, using cutting compound by hand or machine. Wax it last with carnauba, any car polish will do the job.