I re-did the underside of my 1966 car in about 2009. What i noticed was that where the bituminous coating was intact, there were no signs at all of rust. The original brown rotodip primer was still intact and i did not remove it.
On the other hand, along the sides behind the wheels where 40+ years of throwing road debris up had worn away the bituminous coating, there was clearly signs of previous surface rust that had been cleaned up and simply re-primed and painted. There were chips and scratches in that paint that had surface rust. These areas i cleaned up, re-primed and recoated the whole underside with the bituminous coating.
My summary conclusion was that the original rotodip primer and coating system is good for more than 40 years of use as long as it is maintained intact.
As an aside, there were also areas immediately behind the engine bay, particularly behind the diff, where there was no bituminuous coating left, having been dissolved by oil leaks over many years. The presence of the leaked oil however kept the rust at bay. Oil leaks are not all bad
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66 Mini Minor sponsored by
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It was a pleasure ausmini. I'll miss all you misfits and reprobates
