$25 for 8 ounces of POR is not less expensive than $40 for a quart of Red Kote.
I have used POR15 in the past and will say that in my experience Red Kote is a superior product in all aspects.
In fact, I cleaned a tank today by rotating is slowly in the lathe after filling it about 1/4 full of BB's, rinsed it out with acetone, and applied the Red Kote.
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If you look at this photo you can see where someone had tried in the past to seal the seam by applying something on the outside. They were not successful.
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I have it up on a shelf above the wood stove this evening to help speed the curing process.
Mark