Gnarly large rust stain on the bottom of my toilet bowl

[NYC_SKP]

1. Typically, toilet bowls are made of porcelain.

I've seen a few porcelainized cast iron bowls, but yours is probably like most others, made of fired clay, and shouldn't be threatened by rust.

Some rust stains can come from iron in the water.

I can recommend products like "CLR" http://www.walgreens.com/store/c/clr-calcium-lime-rust-cleaner/ID=prod1765587-product

And I also like using diatomacious powder, available at good hardware stores and auto parts stores: http://www.mustangsplus.com/xcart/Nothin-s-Better-Water-Stain-Remover-Paste.html

I've had mixed results with enamel sprays on porcelain, I'd try to clean it first.