Prepping 500 Gal. Propane Tank

Fill it with Dawn and water...   that pushes out all the propane and help dissolve the oil that is the smell...  Lower the water level to below the cut line and cut with a thin cut off wheel on an angle grinder or plasma torch...  cut where you will be putting the hinges on....  do not cut everything where you can get to later...   In other words, leave the corners and maybe a tab or two along the long cuts...    Weld on the hinges and door seals....  door seals can be 1/16" steel...  or anything...  they seal out tramp air...  and hold the door in place..

  Plug and Slot welds, on the door seal, reduce the chance of warping of the door...  there are many ideas on how to do this....   

ALSO...  some suggest welding on the Fire Box before cutting on the door to reduce warping of the propane tank when the door is cut out....