You can mix your own "bog" with many things added to resin or gel coat. If I was re gelcoating the bottoms of a cat ( of which over the last 45 years I have done several thousand) I would mix industrial talc powder vith gel coat until I have a "non slump" viscosity This makes it easy to apply (my prefered way is to put it on with a stainless steel paint scraper, as it is easiest to "sculpt" the bog (filler) to where it is needed) I would use talc with gel coat because the main "thickening" ingredient in the original gel coat is talc (the three prime components are resin, talc, and pigment, these form the greatest volume) so by using talc, the relative mass of material that I am putting onto the hulls is some what close to the original gel coat that has been worn away. I use gel coat instead of resin with the talc because after I have faired this "bog" filler back and then applied the finished coat(s) of gel coat, it is very much easier to finish off with out showing "darker" resin/glass colour through any spots in the gel coat where it has been faired to the original surface or where you have wet rubbed a little to enthusiastic and made the gel coat transperently thing in the odd spot.