When i got the frame for my IT250K it was pretty rusty however once it's sand blasted it was mainly surface rust with only little pin holes which i got away with some good quality fibreglass filler....Definitely one thing to keep in mind but these days frames are hard to come by so i sort of take the first one that comes..and of course with plenty of photos beforehand. For the YZ490 frame, i would rather be focusing at the brackets/supports of the frame..these frames are well known to have either bent or broken brackets and often it leads to the crack on the frame tubes. It's one of the main reason why i want to replace mine since it came with a lot of defects when i purchased it.
