Epoxy is not going to have the UV resistance of Gelcoat...if you put cabosil in the resin it will be hard to sand but hold up better...if you put glass bubbles in the resin it will be easy to sand but ware away fast...there is no free lunch...unless the bottom is all gouged up there is no reason to use a filler. Again if the color coat (gelcoat) is all that is worn away then just lightly sand the surface with 80 grit, wipe it down good with acetone and re-gelcoat the area...its not paint...you can build it up 1/8" of an inch if you want (or more) and sand it down to suit your fancy. Don't make a mountain out of a mole hill. Use the gelcoat...it will be less steps, match the original better, be more resistant to ware than other methods and we be considerably cheaper. Clean/Sand/wipe/gelcoat/sand/polish or Clean/Sand/wipe/Epoxy/sand/prime/sand/wipe/paint/sand/polish the choice is yours.

Regards,
Bob