back in my bodyshop days [68-72], i was in charge of our rust proofing program. it was a competitive product made by texaco.
our policy was to remove all door panels and 1/4 upholstery to be sure the spray got to where it was supposed to go. the only time we drilled holes was when no access was possible.
when some of my buddies had their cars done, i made sure they got "extra" protection.

boy, did i ever get my azz chewed in the later years when they restored their rides !
seems as that stuff was super hard to remove, especially under the hood !
back to your locally scheduled program.