Okay so time to actually update what's being going on - not a lot required to do fortunately!
I haven't got around to doing the smoke test yet, although it's one of my priorities
When I first got the car I was worried the floor pans were rusted and would need replacing. I used the weekend to properly check out what was going on.
The PO had laid down some sound deadening which had began to rust, and caused the area on the floorboard to appear really rusty. Fortunately there was a layer of paint/rust proofing protecting the floorpan so the "rust" quick scraped off revealing the painted metal beneath it. What a relief! I removed the sound deadening and threw it in the bin, and began working on scraping/sanding the old paint back.
Here's a pic of a before/after sort of thing, where I had scraped most of the stuff off the left hand side but haven't gotten started on the right hand side. I went a bit further with a metal wire brush and some tough sand paper to get rid of anything that I could find that was orange/previous rust remover.
You can see how quickly the little bits add up! A bit of weight reduction...
I ordered a product called "Rust Bullet" (from here:
https://rustbulletuk.com/en/) which claims to dehydrate & treat the rust then form an impenetrable layer. I only got a small 1/4 pint tin as apparently that was enough for 6ft Sq... It turns out it wasn't quite, but I did manage to coat all the areas that had been in contact with the rusty sound proofing. On hindsight I should have ordered 1/2 Pint or maybe even a full pint and gone to town with the stuff.
The product was really satisfying to use. It painted on really easily and then within about ~30 mins had become hard. I put 2x thin layers (as per instructions) over the area and left it to cure which took about 2 hours between coats (sunny day, around 20/25 degrees, low humidity). As you can see, if I had more of the product I'd of painted it up the transmission tunnel and firewall. I'll order another tin and do that in the future.
Both the drivers and the passengers side now looks great, the pics above are from the passengers side which was significantly worse. I initially took the rubber inserts out to paint the holes, but I preferred seeing them silver too so I painted over them. You can see a bit of the metal sound deadening surrounding the transmission tunnel.
I could have probably done a better job removing some of the older paint but speaking to the technical advisor team at Rust Bullet, they suggested it would be fine as long as the Rust Bullet created a complete "surface".
I'm hoping this gives me a couple of years before I need to do something more permanent.