And you know how guys say "Rubberized Undercoating" sucks?
I did the skid plate on my 02 4R with it, and 18 years later (from when I bought it) it is still rust free. I did it on the first oil change at around 1.5-2k miles.
I do not recall what I used, but IIRC, I picked it up from Advance Auto.
I prepped the '02 skid with Brake Kleen, rubbing alcohol then Brake Kleen again.
It has not chipped, peeled or rusted.
So do you all see why all this is tricky?
It all depends on the current condition, prep, and how thorough you are.
I, me, myself would honestly pay $2,000-$2,500
for Toyota to WaxOyl the chassis at the factory. Sure, we can do it for much less, but we are much less thorough due to all the components and body in the way.