>A few years back now Toyota had a big recall
Mustie1, absolute YT gem of a human being, did a couple videos showing his efforts to salvage post recall Toyotas. Recall was a scam. Instead of replacing bad frame, or stripping, cleaning and repainting Toyota opted to pay third party contractor to just spray some black goo on frame rails so they last 12 more months while the goo hides corrosion (and traps more moisture speeding up the process).
RRRUST 2013 https://www.youtube.com/watch?v=9s7N8QEAAeM
toyota 4runner frame rust repair 2013 https://www.youtube.com/watch?v=gBB8YX1I1QU
more toyota frame repair 2013 https://www.youtube.com/watch?v=hdLNKOdi4-A
Toyota Tundra frame repair 2016 https://www.youtube.com/watch?v=5IBXYnYYccI
toyota tundra rusted frame repair update 2 winters later 2018 https://www.youtube.com/watch?v=nn-cqoVYQR0
TLDR involved welding plates to replace eaten metal and regularly coating in oil/liquid film protection every year.