Frame Rust Areas

Good morning.
My 98 Mountaineer is still in pretty good shape, but i might have to run it all winter now. Are there specific areas that i should concentrate on when applying a rustproofing? I know of the rear shackles on these but nothing else.
Thanks in advance.

I do my whole underside with fluid film
Takes 4 cans

If you rustproof just make sure to use a type that does not coat with a substance that is rubber etc. These will allow water to get between the metal and the coating and will be worse in the long term than doing nothing. The oil based coatings like donalds referenced are the best.

I am going to use fluidfilm, but at $10 a can I can only swing 2 cans right now. Thats why im asking what areas are more critical.

if you are interested in protecting the body I would focus on the rocker panels and "dog legs" All 91-01 Explorers start to rust there first

next would be up front around the radiator core support
Then around the rear bumper and rear shackles

Excellent. Thanks again, all.