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Here's the inside view of the first two stiffeners I completed.

For my own reference, 20 grams of the white Pro Seal compound and 2 grams of the black accelerant was enough to do 3 full stiffeners with a bunch left over. Might be enough for 4. However, I wasn't able to start a 4th one before what was left was becoming unworkably stiff. My recommendation is to do 10g + 1g batches of Pro Seal and try to knock out two stiffeners at a time with that.

Here's the outside surface of the tank with two stiffeners fully riveted in. As you can see, the excess Pro Seal cleans off nicely. I'm very happy with how these turned out.

I put a little dollup of Pro Seal on the end of each rivet prior to inserting them. When I put rivet tape over them, the excess squeezes out into a little donut. When the piece is done I just wipe this off with a paper towel before the Pro Seal has set.

Once I peel away the Easy-Mask paper, this is what is left. I'm pretty happy with it; all of the excess is gone but the fillets are definitely solid.

This is what the stiffeners look like after I've applied all of the tank sealant but before removing the masking. In addition to putting a healthy layer of tank sealant between the two pieces and a dab on each rivet prior to back-riveting, I also put a good fillet around each edge and then a dollup on top of each rivet's shop head just for good measure.