Michigan winter frame coating?

[PJOJeep]

In the last photo I think your tub at the body mount needs more attention than the frame.

I agree....mine looked like that on both sides, cut it out and replaced it with ones from Pocono Metal. Remember the inside goes first, so if you haven’t borescoped it yet, I would. The center crossmember mounting point appears a little suspect.

 

[JEEPCJTJ]

And the fluid film has to be sprayed on a bare metal surface? Or the film will Cover existing rust and stop it from continuing?

Either way works. Fluid film likes to work its way into any little space it can get. I think it actually sticks better to surface rust than bare metal. It does like to work its way into big rust flakes too but there's no way to tell if it's covering everything under there without removing them so if you're going to remove them it should be done before the FF.

Any rust beyond surface rust can't be good.

 

[Goatman]

Fluid Film is almost ideal for your purposes. It can get into areas that paint can't. And it is removable with little work. And it coats and protects through the rust keeping the rust from getting worse. In the Spring you can wash it off and get at the rust in warmer weather. A good coating underneath getting inside the frame and torque box will keep the corrosion at bay.
Por15 is a good product, but 100% prep work is needed to stop rust underneath it. And cold weather makes the prep work difficult without a heated shop for the chemicals to be effective.
FF is lanolin based. The stuff that keeps all those sheep warm and dry in the wet and cold weather. Plus if you get any on you just rub it in to help with your chapped and dry skin!