Acetone can soften the gelcoat and lead to longer term gelcoat damage. I NEVER use it. My first choice for any stains on gelcoat is oxalic acid. The best form is in a gel. FSR by Davis Instruments is a great choice. There are others out there . If that doesn't do it. I use MEK. It is less damaging to gelcoat. As always, use with moderation and lots of elbow grease and ventilation.