Are you asking about the 303 Aerospace Protectant or the 303 Cleaner? I use the cleaner all the time on transom soot and things like bird droppings,carpet stains, vinly dirt, grease, etc...fantastic stuff. Aswell, we get great reviews on it from customers. We now carry the Industrial Strength Cleaner, basically just a stronger version of the 303 Cleaner.
As for as the 303 Aerospace Protectant, I have read some information on using on gelcoat and it is supposed to work well, but to be honest, I do not think it is a strong point of that product. I do not feel it protects gelcoat very well, just my .02. Now.....upholstery (vinyl, leather, plastic,etc) it rocks though...very good stuff.
We are dealers for 303...if you want to get a little info about the products we carry just visit our site at this link...