For the angle grinder method, where the dust is really unbelieveably bad, find someone to lend, or hire a positive pressure hood and visor. This provides fresh filtered air from a battery powered fan and filter normally worn on your waist and a flexi pipe to the visor to supply the air. When working in the tank the pipe is just long enough to position fan/filter outside the tank. Advantages are you can breath fresh air, you can see better, not so claustrophobic, and it keeps your face cool. It's still not a good experience but it is much better than without. Be prepared to spend a long time cleaning the kit before you give it back to its owner !