ProSeal that has been "proportioned" is available from Van's and ACS (among others).
When you are ready for the final install of the tank ribs and the tank skins, you will smear a layer of tank sealant (proseal) between the parts as you put the parts in place, hold with clecos, and then flush rivet. The proseal will ooze out of the dimpled holes (this is a good thing). When you jab a flush rivet in the hole, the proseal that ends up remaining in the dimpled hole prevents the rivet from seating completely flush with the skin surface. So, a slightly deeper dimple is desired to get the flush rivet to seat flush with the proseal (desired to prevent a leak).
I have also found that these tank dimple dies work well on the underlying dimpled holes of lap joints in many areas of the skin joints. This aids in the "nesting" of the top and bottom surface sheets.