I agree with Paul that Barkeepers Friend is one of the best (and CHEAPEST) all-purpose cleansers.
Barkeepers Friend (regular powder) & “FSR” Fiberglass
Stain Remover both contain Oxalic Acid,
Barkeepers Friend “Lime & Rust Remover” contains Sulfamic Acid, and Hydroxyacetic & Citric Acids.
Fibreglass has “excellent” resistance to all the above acids.
Caution: Fibreglass has a “poor” to “limited” resistance to Phosphoric Acid (includes "Naval Jelly")
, which is not really a rust “remover”, so much as a great rust “converter” (iron oxide to iron phosphate).