As Chris suggested, I epoxy both sides of the flange that will be adjacent (inaccessible) after welding.. For cleaning out the spot weld holes, I take an extra drill bit the same size as I used for the plug welds and flatten the end using a roloc sanding disc, 80 grit or finer..
Then use the sander to back-face (mimic an end mill) the back side of the cutting edge, leaving about a 1/16 or slightly wider flat surface..
This will limit the amount of surface area of the flat so it is more effective in cleaning out the epoxy from inside the hole..
Ignore the color of the epoxy, it should be black...
Despite the heat from welding, note that there is minimal if any discoloration of the epoxy in the vicinity of the weld. You will see better results the longer the epoxy cures, so if something can sit for a week, great! After welds are all dressed, I'll normally use more epoxy and a paint brush, wiping the brush against the edge of the flange to wick the paint between the two panels. When it runs out the other side , it should be sealed well.