I second Jentine's suggestion of Whink. It is designed to remove rust stains from fabric
. Works great on fiberglass
also. Available in grocery, hardware
and good marine
"Rust Stain Remover in the "brown bottle" has been the #1 pure liquid rust stain remover since 1947!!!
This convenient liquid formula quickly and easily removes rust stains from white sinks and white toilets, as well as colorfast fabric