If you plan to remove all the gelcoat you could use an angle grinder: try and get a concrete grinding disk, it will not clog and removes the material with less effort.
However, if you only need to clean out some superficial cracking in the gelcoat use a Dremel tool with a steel cutting cone, this will widen the cracks and leave a good 'V' for micro-balloons.
I hope this helps
Andavagoodweekend Old Salt Oz /forums/images/icons/cool.gif
Have seen an electric smoothing plane used quite often for this purpose. You can adjust the amount of shaving to cigarette paper thickness so you are always in control. Have also just experie3nced it done by sand blasting...very messy.