Summarize:

If you happen to get your  fiberglass cast dirty, try wiping off the dirt, food, or other mark with a damp cloth. Make sure the cloth is only damp and not too wet. It should not leave any pools of wetness behind on your cast.   No matter if you have a plaster or fiberglass cast, you should never get your cast wet or submerge it into water, even if you get it dirty. Though fiberglass casts are waterproof, the soft lining inside is not, so you want to keep them dry.  If you have a fiberglass cast and a waterproof liner, you may be able to get the cast wet. If either type of cast gets dirty on the outside and a damp cloth isn’t enough, try using mild dish detergent. Put a small amount of dish detergent on a damp cloth. Gently and carefully wipe away the dirt. Use a dry cloth to wipe away the soap and dry the cast. The most important thing to do while you or someone else has a cast is to keep it clean. This means keeping it away from dirt and sand if possible. You should also try to limit how much you sweat in it, because sweat and grime can cause it to get dirty. Be careful when you eat. Try not to drop food onto or into the cast. If you need to, cover the cast while you eat. This may be important if a child has an arm cast.
Wipe the outside of the cast with a damp cloth. Use mild detergent to clean your cast. Keep the cast from getting dirty.