Again it depends -- because of the residual sludge  which adheres as a

  Some welders will  - if the tank is throughly cleaned with steam and

 Some welders  don't seem to give a darn - and will tackle anything --

 Some welders will fill or half fill the tank with water - after

  I suppose it depends on how much risk you and your welder are willing

  I suppose the risk would be less if one drilled a pilot hole and then

  By the same token  when a fuel oil tanker turned over here on the

Adding sumps and inspection plates .. Again it depends -- because of the residual sludge which adheres as a lining to diesel fuel tanks is highly flamable most welders will not touch them .. saying nobody welds on a bomb and them that do are idiots ... Some welders will - if the tank is throughly cleaned with steam and detergent and steamed again .. and they are satisfied they are really clean . Some welders don't seem to give a darn - and will tackle anything -- I've seen them do it and it scared heck out of me to watch them ... Some welders will fill or half fill the tank with water - after cleaning - to work only on the top of the tank - such as to weld in a new fill or vent pipe ... I suppose it depends on how much risk you and your welder are willing to accept -- I'm with the top lot .. I suppose the risk would be less if one drilled a pilot hole and then used a saw zaw or the like to cut a hole for an inspection plate AND filled the tank with water -- but it would still involve some risk ... By the same token when a fuel oil tanker turned over here on the Balto Beltway and tied up traffic for hrs because it could not be pumped out by any of it 's normal openings -- a fellow climbed to what was then the top and cut a hole in the aluminium skin with a cold chissel -- must have had 100 people watching him that day at least ... brave soul that he was ... don't know how the tanker truck was repaired .... but am sure that it was .... All the Best Ken m/v Mrs. Hudson