nice work hammer . . . you're almost there . . . Don't use the belt sander!!!
The straight line is awesome if you have a big air compressor.
otherwise fairing board and block sand the piss out of it. It's gonna take a while. also if you have a little 1/4 sheet electric sander you can do some nice finish work with that . . . the trick is to keep it all flat against the transom . . . otherwise it'll show.
it's great for skimming and filling the pinholes with razor blades . . . if you have large areas to build up then it's not the best product.
If I were in your shoes . . . I'd make a mold (like a veneer to cover the face of the inside of the transom . . . that's the fastest and easiest way to skin that v20 . . . then you only have the back side to finish.