Probably not much more than you're doing. Remember, the rivet detail and such is mostly in the gel coat, gel coat is nothing more than a paint coat in the mold basically so it will come off like the paint does. A chemical stripper might work better for you, but I haven't personally tried stripping paint from fiberglass, anytime I've removed it, I've done it the old fashioned way, elbow grease and lots of sandpaper.