whatever you do, you'll be uneasy. The goal is only to reduce the combustion area without making it impossible to weld, due to too much heat transfer. I use soapy water and still have all my body parts (whether they all still work properly is another iussue...). I give tanks a good wash first, as you suggest.
Adrie's comment is interesting - in Australia, petroleum is petrol...
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.