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I have to drop the tank to check it out. Only leaks when tank is full! What are the odds of a used tank being good? Anyone had a good experience with getting a rad shop to resolder a leaky tank after it's been steamed out?

You're kind of on your own researching local services, or playing games with shipping.

I've heard of filling them with an abrasive, shaking them to make unbelievable noises, then coating the inside with epoxy. Any repairs take experience because of the old gasoline fumes factor.

If the tank is out you can check the mounting brackets. I had an oldie let go from rust while filling the tank. There was a crunchy thump and the filler neck stuck out the door.

Well I just had a new exhaust system installed and decided to fill up the tank. Went to clean the window and noticed gas was pissing out on to the hot muffler and tailpipe! I drove the few blocks to my garage and shut the car down. .Searched for a piece of rubber tubing to make a siphon(Arkansas credit card . Had to find as many jerry cans and containers as I could to hold the 70 litres. Mouthfull of gas. Guess I'm lucky it didn't blow up or I'd be orbiting the planet.

Especially given you location and the year of your car, it may well be the tank. But I've had more problems with the evap hoses. They get old and hard and leak, sometimes fluid even. But if it is the tank, they tend to leak at the seam, just below the seam and at the base of the filler neck. I've welded up a few tanks over the years, without steamcleaning them even and I'm still here making noise. I've seen some epoxied with some success and there's even stuff you can put in to seal them, BMWs are far from the only tanks that rust and are hard to find. The seam is the favorite place to leak and it is tricky, check with local shops, radiator shops and see what they say. And if you're lucky, maybe really lucky it may only be evap hoses.

My tank leaks somewhere where the halves meet. the rear wheel splashed water and mud on it for some 35 years. When I fill over 50 liters, it smells like gas and the seam of the tank gets wet, even drips.
I've seen on the E24 an extra piece of metal covering the gap towards the tank, prolonging the wheel well - seems very useful.

I had the exact same problem. Filled it up once last Fall and when I arrived at home, there was a very strong fuel smell. Went around the car and voila, fuel was slowly running down the side (RH side) of the tank. I haven’t figured out where the leak is because it’s all dry from sitting during the winter months. I suspect it’s either at the base of the filler neck or along the seam.

I just installed a new tank last Sunday. Sadly I didn’t have a choice to go that route. Back in the late ‘80’s, my Dad had the tank repaired by a shop with what appears to be fiberglass upon close inspection.
I have also heard of places using some type of epoxy too! I believe that this may be the most common process for this repair. It would definitely the most cost effective, that’s for sure. A new tank is big coin and I still added three coats of rocker guard to help protect it.

I always put in a new tank. It's $360 at the dealer. The Euros all need it. The base of the three vent pipes are always toast. You need to trim the trans ocean hold downs to fit the new one. The new tanks are a skosh different. There's these tabs, that, get in the way of the trans Atlantic hold downs. You send the helpers home, while you trim steel off of the car. Or, you can cut unnecessary seam off your new tank. Bing, bang, boom, you're back on the road again.

On 77+ cars E28 tanks will fit, are the same as 528i tanks and common used. For 75/76 no such luck.