I don't think that there is any SAFE and CERTAIN way to do this, short of:

a. Removing the tank
b. Having it cleaned (of all volatiles) at an industrial shop that does this sort of work
c. Welded AFTER a+b.

I myself could not trust any other repair.

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86 560SL
With homebrew first gear start!
85 380SL
Daily Driver Project

http://juliepalooza.8m.com/sl/mercedes.htm