My Lord! Is this the first thread on S2ki on how to deal with rust? If so, congratulations to the OP and good luck.

In post #1 OP asks if this rust is really urgent or not. Rust is always urgent.

I have a '59 MGA and a '57 Volvo, and on the boards dedicated to those species, rust is the prime topic of concern. The MGA's, in particular rusted from the inside out. Cut it out and fabricate new parts is about all we can do with those cars. The oil leaking from the engine is the only thing that saved the frames until now.

First, WD-40 is not a penetrant. Get something like PB Blaster or Krols. High heat is also very good at loosening stubborn bolts. Heat with a propane torch, let cool before working. Get a breaker bar for your sockets. The breaker bar can make a big difference. I use WD-40 if I wash parts in soap and water. I air spray with a compressor, than shoot WD-40 onto the metal to prevent flash rust. But only for surfaces not to be painted. But motor oil is actually better and I store unpainted, non-aluminium engine components (for example, the face of a head) coated with oil.

You have many different challenges. If you can take parts off and blast them, that would perhaps be the easiest. But if working on and under the car, you are likely to need an assortment of strong wire brushes for the angle grinder and well as the drill. And sanding wheels and grinding wheels. And a full face shield!

You really have to get all of the rust off before any paint - including POR-15. For some small holes in non structural areas, you can fiberglass rather than weld. But even for fiberglass you need to treat all of the rust and apply to a clean surface. On my Volvo, the floors looked like they had a skin disease. I had to really wire brush the bejesus out of the rust - multiple times until it turned from brown-orangey rust color to black. Then treat with the phosphoric acid rust converter - keep it wet for about 30 minutes. I used a coarse Scotch-Brite pad to work the converter into all the tiny holes and crannies. Then clean with a solvent or non-greasy cleaner before applying the POR, of if not using POR, a primer.

If you wear glasses and are working on your back, protect the lenses from the phosphoric acid.

Protect your eyes when using those wire brushes. On the angle grinder in particular, they are violent.

I also used an internal frame coating from Eastwood Garage in the inaccessible areas. This is essentially a very fluid rust encapsulator paint (Eastwood calls it Internal Frame Coating) that you spray into the inaccessible recesses of the car. I used it in the the sills and in the frame. It is applied with a long flexible tube that you snake into the area through drainage or other holes that might be available. For areas that are accessible, but hidden, like behind the kick panels and up under the dash and inside the doors, I cleaned, wire brushed and sanded as much as possible and then used a rust encapsulator paint.

POR-15 is quite good, but a difficult product to apply correctly. There might be other products that you should consider that are of equal quality.

There is a guy in Upstate New York - the rust capital of northeast USA - who posts videos and has a facebook page under South Main Auto Repair. He apparently likes the Fluid Film, but not for covering up rust - just preventing it before it comes or after it's gone.

Good luck with your project. My Volvo is still a work in progress.

 

Having lived in Vermont most of my adult life, (after a brief stint up in Canton New York in my late teens early twenties), I have to challenge this claim. Vermont has it all over New York on this front.
Not even close.