I bought the 1-13/16ths socket, was going to clean it while leaving it in but then I couldn't break the strainer nut loose. The ones I've seen in the vids had a 19mm bolt head welded on somehow because space is tight and now I get it. There isn't room for the socket, the 3/4 to 1/2 adapter, and the head of a socket wrench with the axle shaft in the way. I tried with a vice grips on the adapter, finally got it situated so I could really pull but just would not break free.
So I'm all Eff it, I spent the $40 on the socket, I want to see that strainer so I took the tank out. More thorough job anyway. Wasn't all that tough. The strainer looked remarkably clean.
That's just how it came out - the rusty bits on my finger are from swiping my finger just inside the hole, trying to show two things in one photo. Also much evidence of the same sort of rust seen in the primary filter on the nut on the hose connecting the strainer to the fuel line:
That didn't clean off all that well, looks to be rusted itself. I'll get a new one.
Looking through the sender hole one can see rust on the bottom of the tank but a seat of the pants examination of the outside of the tank indicates that it's far from seriously weakened. The metal thumps pretty solid.
I have an electric pressure washer, not nearly as strong as the gas powered varieties but it will put out some pressure. So far my searching indicates soap and water followed by an acid bath somewhat stronger that vinegar.
I see no swirl pot inside. I can see the hole where the strainer attaches, this looking through the sender hole. Nothing resembling that. My 325i has one. I might see if I can get a recommend on a service around here that would clean and then coat with some epoxy solution. The objection to an epoxy coating earlier involved a Bakelite coating on the swirl pot.