Use acetone first. If theres any wax or other stuff it won't get sanded INTO the gel coat. use plenty of acetone.
Acetone,sand,acetone again, then I used a slightly damp ''white'' clean rag~rinse with water & do it again, and if ya want use acetone again, go ahead it won't hurt.
If ya have serious trouble with stains, use ''ON/OFF ACID''~~ be sure to wear a good heavy rubber glove. It will burn ya, turn grass brown, smoke, and make ya Gagg!!! Thats what I did before I painted my cats... 3 of them and they look great.........I did use a 80 & 100 grit- 3 coats of paint, no grit marks.. It won't hurt to try 180 grit, see what it looks like !!!! GOOD LUCK!!