Yes the red is some kind of oxide primer. It says it prevents rust to advance, when applied to metal. I have been using it as a primer to the bare steel after doing welding. On top is just multiple layers of rock chip preventive spray.
Both of these products are spesific for my location in...
I did mine last winter, both sides. But only these areas where the floor meets the bulkhead. Relatively easy place to do, just have to remove the floor mat inside and protect the wire harness from heat.
I love the handling of this car, because I can steer by gas pedal. It handles like 4wd on icy curves but I can get the rear out quite easily. It's still very controlled with nordic studded winter tyres.
That is a find, 53k miles in 20 years!
Sport suspension is very noticeable when driving. It sits firmly on the road and you can take corners with minimal body roll.
You are right, it's -25 celcius here currently, been like this for a week already. But that rear door has been like that regardless of the temperature. Other doors work perfectly, I'm using teflon spray for door hinges.