Update: Its been approx 10 months since the first application of RP-342. Including one winter. The rear wheel wells and everything in the area are the trouble spot for rust on this truck. Nothing rinses off under there. As stated before, my prior Fluid Film habit hampered the first application. Lots of oily soot, sand, and grit flaking off in chips exposing areas of rust underneath. Would appear its still an issue.
One area that still has minor FF flaking off. Still has soot clinging to the body. Same for the other side.
For the most part, the RP-342 held up pretty well. Used a pressure sprayer on low to get anything loose and reapplied. Decided to leave the rusted surface metal bare and coated it with nothing but RP-342. See what happens next year.
In summary there were a few spots that chipped off and some minor rust bled through, and thats probably my doing not getting all the FF off. But this time around was fast. Other than these areas the frame and undercarriage is clean.
I didn't reapply everywhere. The rear still holding up from the original application. Going to leave it alone.