there is an assortment of proprietary "metal treatment" chemicals that can remove the rust stain. In addition vitamin"c" on the spotthe same as the vitamins that a person would take. Dry acid in a cotton sock and "swabbed " in and around the area could also do the trick, careful though you might want to pre-test a un-seen area that the "swabbing" may affect the construction material
form_title= Fiberglass Pools form_header= Relax in the water with a fiberglass pool. Do you want a salt water pool?*= () Yes () No What size is your backyard?*= _  What is your budget for a pool?*= _ 
Yes the low concentration of salt in a salt water pool would not ham fiberglass any more then much stronger concentrations of salt in sea water water harms the hull of a fiberglass boat.
sand it or grind it out depends on what finish you want
you would have to go to the store and buy some bleach and put it in the pool.
The white film on the sides of a fiberglass pool is likely the result of mineral build up. You can find products at your pool store to help remove this build up.
You have to remove the water or the land fill in order to work on the walls of the vessel.
Not recommended. If the fiberglass pool floats, it will do considerable damage.
No!!! Under no circumstances should you drain a fiberglass in-ground pool. Unlike the standard in-ground pool, the fiberglass pool base is unreinforced concrete or other hard surface material that was applied directly to the soil in a thin layer. It is only there to allow the fiberglass to be sprayed on and form a hard shell. The weight of the water is what holds the fiberglass in place. If you drain the pool without refilling it immediately, you will allow external ground pressures and/or ground water to buckle the sides or bottom and you will have the fiberglass completely redone.
What type of existing swimming pool? You can patch an existing fiberglass pool.
If the pool is empty and dry, vacuum it. If filled with water, use a sieve to seperate the grass from the water. You must relise that this will not remove ALL pieces of grass from the pool but it will be mostly clean.