No contradiction there as you apparently have a vehicle with undercoating applied when new. OP has scraped the undercarriage of a 60 y.o truck - and we are not sure to what extent. If he undercoats properly prepped metal he should have little to no worries. If he applies when there is crud on the undercarriage he would be better off saving his money and washing every so often as advised. Same as if we were talking about paint on the surface.
