Now the ospho residue is gone the surface is ready for paint. I went with a zinc metal primer just on the off chance that the pinholes *were* due to some sort of electrical activity.
Zinc is more vulnerable to that stuff than steel so it'll get eaten up first and give me a bit of a buffer to notice weirdness and remedy the situation before it gets after the tanks structure again.
Tape off the nozzles where the hoses connect so it doesn't get into my drinking water, and spray the heck out of all the weld seams and any part that had corrosion or pitting that I cleaned up.
Now we hunt for the actual leak. I blew into a hose connected to one inlet and covered the other with my thumb and listened for hissing. This proved difficult so I started dousing suspect areas with soapy water while blowing air into the tank.
Bubbles revealed a small pinhole leak adjacent to the area that the previous owner had gooped with epoxy.
I removed the zinc coating from the area around the leak and mixed some drinking water safe epoxy putty.
If you leave the paint the epoxy will stick to the paint better than the paint sticks to the metal and it'll just peel it off.
The epoxy putty is like a stick of play dough, the core is one part the outer layer is another. Break off the appropriate amount, work it around in your hand (GLOVES!!!) until the 2 parts are mixed and then slap it onto whatever quickly before the chemical reaction kicks off and makes it too hard to stick to anything.
Wait for the epoxy to cure, go to the hardware store for the right size hose because I bought the wrong stuff before
Drop the tank back into its crevice
Double hose clamps are always a good idea where feasible.
I preemptively wrapped the hose barb adaptor threads with PTFE tape. I call it Teflon tape, grandpa calls it Teflon tape, it might actually not be Teflon.
Plumbers (PTFE/Teflon?) tape isn't sticky. It's like a thin soft ribbon. It squishes between mating threads to make them water tight.
Make sure you wrap it the right way so that when you're screwing in the threads it pulls the tape tighter instead of unwrapping it. Otherwise it will be a pain in the ass and won't work properly.
Last night I put 5 gallons of water in the port tank for drinking/cooking/dishes so I have to wait until that runs out before I can finish plumbing the tanks together. Maybe 2-3 days.
You can see the mess I'm replacing. Garden hose, rust balled hose clamps. Terrible. Probably not safe for drinking water anyway.