Much of the shot used will have stuck in the really hard muck found in all old tanks and be damn near impossible to shift.
Frankly I'd look for another tank as the possiblities for major disaster are legion.
Even more frankly I'd never buy a bike thats had its tank shot blasted, as likely enough the fool who had it done had no idea at all how to go about things, seen too many bikes wrecked like it.
These idiots give parts to the blaster that are not degeased, never masked up, I've seen complete wheels bearings and all go to the blaster.
One idiot I knew went to a place that shot blasted and stove enamelled parts,then came back moaning that the guys has not stripped the swinging arm off the frame and it was locked up.
When I asked the idiot why he did'nt strip it before hand, he replied 'he didnt have the special tool needed' ..................as if the bloody shot blaster would have.
The trouble with old bikes is that there are too many ignorant idiots messing with them.
Just another reason I'll never buy a "restored" bike unless I know the sellers capabilities.