I've got a metal grill that I bought used from here, it was black with rust coming through. I decided that I'd get it powedercoated along with the wheels.

Now that hasn't been a year but I've noticed rust coming through the coating? Is this to be expected? I assumed that once blasted and coated it wouldn't re-occur?

It wouldn't cost much to get it re-done but the question is should this been done for gratis or treat this as annual expense? ( or buy a plastic grill lol)

It depends, like anything, on the quality of the coating. A good coated item won't rust, a poorly coated one will.

If it was properly prepared like striped and blasted it shouldn't really rust through again, having said that, being a grille it's more prone to chipping from stones and the like which obviously won't help.

The rust is spring up mainly on the back of the grill (engine side) and where there are any metal joins.

Worth asking for a free redo then?

We have a few bits & pieces built at work for harsh environments, and we'll get it zinc plated before the powder coat goes on for an extra level of resistance.

The other thing is that blasting doesn't necessarily remove all the rust, and like a weed, if you leave the root in the ground it will come back.

A blast, rusteater, then rust converter before a zinc primer/ zinc plate might be a more complete way to do it.

I had some steel MG wheels done professionally and had the same experience. The rust was back within about 12 months.