Hi. I want to buy my first flat fender soon and from what I've been reading, the condition of the tub is really important. I know that if a tub is completely rusted you might have to replace it, but are there hidden places or specific spots that basically make a tub ruined - like if it's corroded at the spots where the tub attaches to the frame? The one I'm looking at does run, I'm just not sure about other deal killers I should be on the look out for. Thanks!