[danthony]

Unless you're talking about enameled cast iron (Le Creuset, Staub), then acidic ingredients like tomatoes will remove the seasoning from your pan. It shouldn't hurt your sauce, but re-seasoning your gear is a pain in the ass.

[ubercore]

Not really, in my experience. I had a rusted pan I pulled out of the basement, spent 20 minutes scrubbing and rinsing, a couple of rounds in and out of the oven with vegetable shortening, and was non-stick enough already to easily cook eggs. Am I missing a step I should be doing?

[bhousel]

Wow, you +1 for teaching me something new. I've been using stupid vegetable oil on a cloth all these years to season my cast iron, and it's the biggest pain in the ass. I never even thought to use shortening.

[ubercore]

I think I just read about it on a blog, and I'm pretty new to cast iron myself. But from what I understand, any fat should work.