Just suffered thru this one, so here's my suggestions.
Evaluate your tank before you determine your approach. I had a seam leak but a "perfect interior" which was totally ruined by the boil out. A sealer woud have been my best apprach, but I wanted the leak brazed and ground down.
I was wrong.
Considerations:
1. BMW national parts (Dealer) availability is currently (2) in stock in USA at ~$750
2. Slightly better pricing (I believe) thru a couple folks here that do BMW Mobile tradition - search on "Blunt"
3. Make real sure you are getting the correct tank for your car and that the current filler tube is correct for what you are buying.
4. Check the tank seam very carefully- if you DON'T have leaking or rusting nearest the passenger wheel well where the OEM seal was too short, I would be shocked.
5. Radiator shops can fix, but beware that if the inside of the tank looks good now except for the pin holes, it WON'T when they finish "boiling it out"
6. POR gas tank treatment usually works if done meticulously.
7. RENU is similar product done by shop with similar expected results hopelfully guaranteed for some time period
8. Used tanks may not be any better than what you have unless seller has really scoped it out - and shipping is pricey for long hauls - maybe Greyhound bus freight is cheapest.
WHAT I DID:
9. Vinegar worked very well to clean out the rust and scale left over from boil-out, filling with oil and using wd40 didn't stop near-immediatge rerusting. VALUE ADDED- I lost 5 lbs aggitating the tank over a 3 day period and filtering/screening out 5 gal of vinegar and reusing until all particles and rust were gone
10. Used 1 gal of Concrete Etch (Home Depot paint dept) based on a nast acid- more aggitation until fully coated and turned all rust to dark gray. About 1 hour was all that was needed. BE CAREFUL, this stuff is caustic and will BURN. I wore poly gloves, long sleeves, and face protection.
10. I filled with 3 gal of gas and again and again and again aggitated and filtered gas until NO MORE FREAKING particles or rust (now black) coloring was apparant. ( about n30-40 fills, shake, filter, re-fill, shake etc ad nauseum)
11. Installed tank with stainless steel bolts, flat washers and nylon lock nuts after coaing with blue loctite, used a double strip of door seal from (yes Home Depot hardware area) Also coated this with the electrical silicone to assure good seal - determined tank bottom was bumped in and would not accomodate fuel sender
12. Used user end of small baseball bat and a 5 lb maul to beat the dent out carefully as one can do with a 5lb maul and a baseball bat and lots of pent up anger towards this D**MN tank.
13 Installed the sender - nice fit- used electrical silicone paste on the OLD "O" ring seal - (buy a new one from Blunt)
14. Added 5 gal of new gas to the tank
15. Changed to new gas filter at engine
16. Fed used gas to junkyard dog that can't get enough of this stuff........just kidding.
17. Drove the car to the gas station,filled up, checked filter NO RUST or particles - YEAH!
18. Did novena to Saint Anthony - patron of los causes - just in case!
19. Drive the d**mn car.
Hope this is helpful, or if not helpful, at least amusing!
ANYBODY_ Please chime in if I missed anything here!
Dave in Baltim02e