I recognise a problem!!! I have used a screwdriver to chip away at small runs and non-stick scouring pads on old old stains (3yrs +) with some success. Even on smooth gelcoat it's difficult. I have re-painted with non-slip and had no problems with it bonding. I guess an adequate soak in white spirit might eventually soften it but have hesitated to subject the surface to prolonged treatment.