I would use bondo on the fiberglass instead of spot putty. Of course if you want you can use fiberglass, because it mixes the same as bondo.
When I painted my mustang I used Martin Senior Base coat/Clear coat. For the primer I used 2k Urethane (Finish 1 product from NAPA). It sands great, and fills scratches. I'd sand it with 400 grit, and final sand with 600 grit on the primer to make it as smooth as possible. Of course use heavier grits on the bondo like 80 grit, or 120 to 180... then go lighter. With all of this though, remember to use a good sanding block/board.
If you do it right, you'll get all sorts of compliments. Plus your mustang will look good... Especially if you sand the clear coat with 1500 to 2000 grit sandpaper then buff it. If you did all the bodywork correctly, and taped it up just right... it will absolutely look just as good as any $3500 to $5000 paint job.... Especially if it's cut and buffed for a mirror finish.