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Originally Posted by Ayles

Not sure if you have this option locally, but I just got my oil tank back from Pacific Oil Cooler and they bead blast and test. They can tell you if a repair is needed and if it's fixable. They also do an amazing job at cleaning and painting the tank.

See my thread on it here:
http://forums.pelicanparts.com/porsche-911-technical-forum/770954-oil-tank-unboxing.html

What did they do to the inside?

I have seen tanks that are very nicely restored on the outside, that's full of rust on the inside.