With the ATF I use a plastic spray bottle...used or new.

Run the vehicle up on ramps...usually the back...so the oil will run down the seams in the doors...rails...frame...etc.

I usually get under and look for ANY holes...espec rust holes...a spray INSIDE everything.

The oil runs down into the same places the water does...if it's there first...the water is SOL.

I do this where some drips on the ground won't matter...clean up what you see outside after a few days.

Once you oil in the fall...it usually takes very little to refresh some areas each year.

I've used solvent based paints as sold by Sears and JCWhitney that are supposed to stop rust. The Sears stuff was sold thru their car restoration catalog and was the best stuff...maybe $35 a quart. The other is almost as good...neither works 100%. Main thing is prep...getting ALL the scale loose...espec any pitted places. If you have water getting in from behind...where you have a hole in the metal...a pinhole...it won't work.