Look over items that are missing plastic. Turn plastic chairs around to the back, to repair cracks first, before replacing a missing piece. Place a piece of fiberglass cloth--the woven kind--on top of the cracks, cutting in sections to fit the crack, if small.
Glue around the edges of the crack, until the fiberglass cloth becomes transparent. Administer an accelerator spray on the cloth to help the glue absorb and dry faster into the cloth. This will make the fiberglass become a permanent bond within any hairline or wider crack and adds strength.
Use Gorilla Glue in the center portion on a stress point of plastic. A stress point would be door-strap brackets, or anything that may be movable. Use the Gorilla Glue in place of Crazy Glue over the fiberglass cloth, to create a more flexible bond, which also fills missing space in plastic. Make sure to saturate the fiberglass cloth until it turns clear, so that it will bond with the plastic.
Use putty epoxy made for plastic to repair chunks and large pieces of missing plastic. Use the molding putty epoxy kind that you can mold by hand and use enough epoxy to cover the size of the missing piece. Once the putty is molded and activated, shape it into the missing section, and attach. Let the epoxy dry and cover with fiberglass cloth, for extra strength, and to seal in the repair. Use the Crazy Glue over fiberglass cloth for non-flexible sections and the Gorilla Glue over fiberglass cloth for movable plastic sections.
Scrape away excess epoxy or fiberglass with a putty knife before it dries fully, being careful not to disturb the repair as it dries. Let dry, and make cosmetic changes to the plastic, so the repair is not noticeable. Use either eco-friendly spray paint made for plastics, or regular paint for plastics. Pick a color that matches the plastic being repaired and apply one coat with a clean paint brush. Let dry, and apply a second coat, if needed.