Seem sealer, yes.
As to coating, just a thought, a brain fart, maybe. What about using heavy duty bed liner on the underside of the fenders and in the wheel well? Maybe somebody else here will have a reason why that's not a good idea. I've never done it, but it seems like it might be a good idea. It's designed to take abuse so it seems it would offer protection to those parts. And since it has a textured finished, seems like it would not be hard to apply.
I have a wheel barrow here that is showing some rust on the bottom. I'm thinking I may give it coat of some Rustoleum primer the spray it with bed liner.