When they get that bad, it's sometimes easier to just colorsand then buff. However, here is another tip/trick that has worked for me on neglected fiberglass.... grab that liquid metal polish (something like Mark-V Metal Brite) and a 9" white wool cutting pad (not synthintic) mounted on a 7" rubberized
backing plate (the kind that needs a buffer nut) and go to town with your rotary set at about 1750rpm. Use medium to hard pressure working steadily it until its gone, spurring the pad often (it might take two passes). The chemical cleanser base, combined with the abrasives makes for a fast finish. It's also not a bad idea to use a diffrent pad on the white areas as the colored, for sometimes the colors will leave behind in the whites. Follow it up with a fiberglass polish using a, preferably 9", 100% lambs wool polishing pad (once again utilizing the rubberized
backing plate and buffernut). Top it off with a sealant and presto -- it'll be 60-90 day nice. For what it's worth, I have yet to have good results on heavily oxidized gels using foam pads.