Tape plastic sheets to the shower stall walls that are not in need of repair and on the floor of the shower. Fiberglass resin is very messy and the plastic sheets will protect your work area.
Sand the area surrounding the hole with coarse sandpaper so the resin will adhere. Wipe the area clean using acetone on a clean, old cloth.
Measure and cut a fiberglass sheet from your repair kit to fit approximately 1 inch larger than the diameter of the hole or crack. For example, if the hole is 1 inch in diameter, cut the sheet with scissors or a utility knife so it is 2 inches in diameter.
Place a clean, empty coffee can onto a small piece of cardboard. The cardboard will protect the surface as you are mixing the resin and hardener in the coffee can. Add the appropriate portion of resin to your hardener, depending on the size of the hole you are patching. For example, mix 1/4 the volume of resin with an equal portion of hardener for a 1-inch-diameter hole.
Add the colorant to your resin mixture, following the instructions included with your kit. Several types of colorants are available and all require a different method of mixing to obtain the color tint needed for your project.
Add the thickening agent one drop at a time until it reaches the needed consistency. Vertical surface repairs require the material to be thick so that it does not drip or run down the surface before setting. Stir the mixture for approximately two minutes with a wooden tongue depressor or putty knife.
Place the fiberglass cloth over the hole. Spread the mixed material over the cloth, on the edges of the hole. Spread the mixture evenly. Add more sheets if needed.
Allow the mixture to harden for approximately two hours at room temperature.
Trim any areas where the fiberglass cloth corners are jutting out, using scissors or a utility knife. Sand the surface carefully, starting with coarse grit and ending with a fine-grit sandpaper to smooth it.
Mix another batch of resin, excluding the thickening agent. Use the same portions as you did for the first batch. Apply the resin with a small paintbrush on the same surface area. Allow it to dry for approximately two hours at room temperature.
Sand the repaired surface area with fine-grit sandpaper. Buff the area with the buffing compound included in your kit. Remove the plastic coverings in the stall and clean your work area.