How were the fuel tanks sealed on the XB-70 Valkyrie?
According to Valkyrie - North American XB-70A by Steve Pace, leakage problems existed.

...engineering changes, weld stresses, and areas where it was not considered important to test-pressurize, combined to create an accumulation of leakage problems.  Brazing repairs to the leaking areas proved ineffective.

To the point that

The rearmost fuselage tank on XB-70A No. 1 became increasingly difficult to seal and therefore was never used.

The mitigation was

A non-metallic material made by DuPont called Viton-B was eventually used to coat the porous areas.

(p. 69)
It goes on to discuss that it still didn't work that well.
Fuel tankage diagram from the book