If you have scale and rust that is peeling off in chunks, there is a fair chance that it is weakened. It isn't just the outside surfaces that rusts - especially at fully boxed areas and at cross member joint where road spray crud and salt accumulate, but all the way around. The only way to totally access any damage is to have it grit blasted. Depending on the make and year, sometimes there is enough meat for significant rust without loss of strength. An early 'vette(or Chevy), for instance, rusts easily and many either need major repairs or replacement. Fairlanes, Mustangs and Torinos unitized frames used to break at the firewall - Mopars,??? and so on......