No, you dont want to use resin to modify something, its too thin, resin is for making things in a mold. Its as thin as milk when its liquid.
Youll want a couple of different things.
Bondo. This is like mashed potatoes. So you canb uild up and fill cavities, it cures in 30 minutes to sandable hard.
2 part epoxy clay - This turns to a rock hard clay, in a few hours, but while its soft, its like clay, and its smoothable with water.
If your going to make a mold, just a hard clay will work, smooth it, then primer it, make your mold, pour a piece, sand and re-finish it to make it perfect, and make another mold.
The best resins are found from alumilite.
www.alumilite.com
They also sell clays, tools, silicones, and they are a very good company to deal with. Much better than smoothon.