If your old fiberglass shower stall is looking worn and grungy, it doesn't have to be discarded. Consider a complete refinishing. Fiberglass is topped with special epoxy paint that can be reapplied over old fiberglass. Go to your local home improvement store or online and buy a fiberglass refinishing kit made for showers and baths.
Clean and Prepare
Remove the shower head and any other fixtures. Thoroughly clean the shower from top to bottom, making sure to get up all grime and dirt. Remove all caulk from the corners, edges and around the fixtures, using a caulk-removal tool or flat-edged screwdriver. Run a folded piece of very fine sandpaper (400-grit) into the corners to make sure you've gotten up all the caulk. Finish the cleaning with a treatment of liquid TSP cleaner. Make sure you have plenty of ventilation in the room and wear rubber gloves while doing this.
Sand and Tape
The shower stall has to be completely deglossed to hold the new paint, but it has to be done in a way that doesn't cause deep scratches to the surface that will show through the coats. Use #0000 steel wool, which is very fine, to buff the entire surface. Start at the top and work your way down. Buff until the surface becomes dull and doesn't reflect light. Thoroughly rinse the dust, then thoroughly dry the surface. Once it's dry, use painter's tape to tape off all border areas that won't be painted. Tape plastic over adjacent walls and floor.
Different kits come with different systems of specialized fiberglass paint. Generally, you're going to apply multiple thin layers of spray paint, either through cans or with a rented sprayer (the preferred method). Ventilate the room with a fan in the window. Put on goggles, gloves and a respirator. Start the first spray coat at the top and work your way down. Lay a very light coat so there is no dripping or streaking. After it dries, sand it very lightly with #0000 steel wool, wipe down the dust with a dry cloth, and recoat. Generally, you'll want at least four to six coats of paint.