Clean the acrylic tub thoroughly, concentrating on the areas that need to be repaired. Be sure not to leave any cleaning solution residue on the surface before repairing. Allow the tub to dry completely.
Open the Plast-Aid Multipurpose Repair Kit and mix a sufficient amount of Plast-Aid powder and liquid using the mixing cup included in the kit. Use a mixing stick and stir well.
Find an acrylic paint that matches the color of your spa's shell. It may take some trial and error to find the right color. Mixing a few colors may be necessary. Add the paint to the mixture.
Scoop out the mixture onto a plastic putty knife and smear on the crack, filling it evenly. Allow the mixture to dry.
Sand the area using the fine-grit sandpaper to make the surface even and smooth again. You should be barely able to see the crack when sanding is complete.