That's cool news about the MSR. Sounds like it might be a good all purpose rust remover. Wonder how it works on greasy/rusty parts?
Also, good to know about the line of sight thing.
Before you do anything to derust a tank, I suggest knocking down the worst of it with agitation. I like a couple handfuls of glaziers points
or roofing nails
. Remove the petcocks, roll the tank all around for a half hour or so, dump it all out, rinse with acetone (no residue, dries quick) then either do the electrolysis or MSR whatever.
I too prefer Red Kote over Kreem but whatever you use, you have to get the goop all over the insides. I do this by letting the sealant settle for a minute, pooling in the tank. Then, flip the tank 180 degrees so the goop is on the 'ceiling' of the tank and drips onto the 'floor'. Wait a minute, then rotate the tank 90 degrees and repeat. The idea is that the goop dripping down is likely to not miss a spot like rolling the tank might. Use an lighted inspection mirror to check it out later if you're really anal about it, which you should be.
Let the goop thoroughly cure before filling the tank with gas. This whole process can take a few days altogether.