None of the normal primers has any rust preventative effects, the
exception is the two part auto epoxy primer (might explain your friend's
rust problem). Standard Rustoleum is a pretty tough finish but not as
good as a powder coat or epoxy based enamels. I used the Rustoleum on my
trailer hitch, it gets exposed to the road salt spray all winter. After
3 winters it is starting to get some serious rust through but I'd
consider that to be pretty reasonable.

I have been using a "machinery enamel" in a rattle can from my local
farm supply store for all my machinery projects (light grey, battleship
gray, hunter green) and the Rustoleum semi gloss black for things like
Jeep bumpers and trailer hitchs.

Alex wrote:
Is RustOleum any good for long term rust prevention?
I just got a can of primer and paint of it from HomeDepot but want to
hear if anyone had a bad experience with it.
I am going to use it to paint bare metal parts of my new metal grinder.
I live near the Pacific Ocean so rust is a big problem here.

Thanks