To bring the bodywork repairs to a close, it was necessary to seal everything up with a variety of chemicals and coatings. I am indebted to skipunda for his advice here, as I'd have ballsed it all up without him.
After welding, the floor was in a right old state. Surface rust, greasy fingerprints, weld spatter, left-over bitumen sound insulation and melted underseal. It wasn't a pretty sight.
What's worse, the rear floor wasn't looking too pretty either. I hadn't stretched my repairs this far back and hadn't anticipated this...
That's the drainage bung hole. I won't show you what the actual bung looks like, but it's not pretty.
So, to get this all scrubbed up nicely, I use an etcher. This is a very primitive method, and there are more sophisticated products on the market for this, but not in Poland. So it's out with the Phosphoric Acid, in an atomiser bottle

and the wire wool to rub it down with
A bit of elbow grease, a go-over with the vacuum cleaner and a wipe-down with panel wipe (or white spirit), and you're ready for an anti rust treatment.
I've picked up plenty of these over the years, but they all work in essentially the same way, converting surface rust into a stable oxide, and sealing the whole layer under a membrane. It doesn't cure rust, but it's an added layer of protection. There's also zinc paint which is good if you've just drilled a hole in a panel (for fitting body kits to sills, for example.) This time around, I used Dinitrol RC900, which sprays out like WD40. Spray it on, let it fizz, then give it a tiny drop of water on some tissue paper to stop the reaction process, in case it eats too far into the metal.
Once it's sprayed on, it should look like this:
And that's the new bung plate from BMW, ready to be fitted in. There's one on each side of the rear footwell, and although I haven't looked under the carpet yet, I bought a second just in case.
To fit them, they just drop into the slot and twist, which isn't much of a seal, so to give them all the help they can get we'll use a brushable seam sealer.
You can use U-Pol or 3m, I'll use my local brand. It goes on smoothly, and application is even better if you cut the bristles down on a normal paintbrush by an inch. This was liberally daubed and smeared over every single area of the resto, including the inner wing, the floorpan welding and the bulkhead. I was too filthy afterwards to hold a camera, so there's no photos, which I'm sure you're gutted about.
Although seam sealer is soft AND strong, a more durable layer needs to go under the car, and with all the floorpan and wheelarch repairs I had to look at my options. There's no Waxoyl or POR-15 over here, and I didn't want to mix engine oil with pig fat and paraffin like your grandad used to, so I bought this:
It's a 2-in-1 seam sealer and underbody spray. The upside is it makes a good spatter pattern of thick rubbery sealant which is similar to the original BMW stuff, but the downside is that it needs a compressor and a special gun to be used. I had the compressor already, and the gun was about five quid, complete with a special tube for injecting the sealant into cavities. This tube turned out to be utterly useless, but the gun itself sprayed a good controllable mist which left the arch like this:
After that, it was just a matter of covering the interior repairs in a good coat of hammerite. You can also use cheap spray paint if the repair is not going to be visible, but make sure it's acrylic so that you have a plastic layer of protection. I'll be covering my repairs in bitumen sheets for soundproofing, so no-one will see them anyway.
With all that taken care of, I can go back to what I wanted to be doing, which is sanding down the doors ready for spray-painting.