How were the fuel tanks sealed on the XB-70 Valkyrie? The Valkyrie flew at Mach 3.1, close to the SR-71's operating speed in cruise. For the SR-71:

Fuselage panels were manufactured to fit only loosely with the aircraft on the ground. Proper alignment was achieved as the airframe heated up and expanded several inches. Because of this, and the lack of a fuel-sealing system that could handle the airframe's expansion at extreme temperatures, the aircraft leaked JP-7 fuel on the ground prior to takeoff. (Wikipedia; emphasis mine)

And if my memory serves me well, and if Discovery channel got it right, the SR-71 needed to meet with a tanker right away because of this leakage.
What was the Valkyrie's workaround/fix? The Valkyrie's Wikipedia article doesn't mention leakage prior to in-flight expansion.
I wanted to initially ask about the MiG-31's fuel tank seals, as the USSR developed heat-resistant rubber[1] (and also special fuel like the SR-71 – the T-6) for that purpose, but since the MiG-31 is a lot cooler, it wouldn't have been comparable, though this piece of information might be relevant.

1: Wings of Russia TV documentary
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