Wipe the surface of the tub dry with a towel. If the tub has a lot of soap scum, wipe it off with a bathtub cleaner and dry it. It is important to start with a clean, dry surface.
Cut a V-groove down the length of the crack with a rotary tool equipped with a triangle-tipped grinder. Extend the V-groove past each end of the crack by a quarter inch.
Wipe off the plastic dust with a clean towel.
Fill the V-groove with fiberglass filler. Pour the filler high enough that it rests above the surface of the plastic. Wait 24 hours for the filler to harden, unless the packaging specifies a longer curing period.
Sand the fiberglass filler flat with the surface of the bathtub, using the orbital sander equipped with 2000-grit sandpaper.