I have welded alot of 500gal propane tanks with no problems or smell... Let them sit put about 3 gals of bleach and fill it with water. Let them sit for about 3 days and drain. Good to go. No smell!
The dozen or so gas & propane tanks I've brazed on I followed a tip from a pro welder & washed them out with a detergent like 409 or Fantastic then rinsed a few times. Never had a pop out of one yet. The cleaning agent really cuts the smell more than plain water.