The following is my experience in sealing my 240Z gas tank, some of this may be useful to someone else.  Some of the POR 15 tank seal instructions are open to some interpretation in my opinion.  Though I used small pieces cut from ziploc bags with rubber bands to seal the gas lines I did get some leaks, I don't know if the acid cleaner ate the ziploc plastic or what.  Maybe duct tape is better for this.  I used a round cut out piece from a vinyl placemat (per someone's post) which worked perfectly for me to seal the sending unit opening though I did have to pay attention to make sure the round cut out was centered.  For preparation I used two gallons of hydrochloric acid and applied it in four applications, 1/2 gallon (diluted with a half gallon of hot water) at a time spread over two days.  Rusty water came out the first couple of times I rinsed the tank.  Hydrochloric acid is supposed to be a substitute for the muriatic acid the POR 15 instructions call for according to the Hydrochloric acid label.  I dried the tank with a hair dryer on low on and off for a week for a total of at least 48 hours forcing air into the filler neck with the sending unit removed because I wanted to finish later with the tank.  Much of the tank got too hot to hold onto.  10 hours into that I picked up the tank and shook it with the sending unit hole down, some water came out doing that.  I also shook the tank several hours after that to move any water to already dry areas to improve drying.  I kept the tank in the garage for drying.  A couple of weeks later I used Marine Clean followed by metal ready, this process took most of a day.  The marine clean was in the tank for 10 hours or more with me shaking the tank followed by turning the tank every two hours.  I followed that per the POR 15 instructions with the metal ready, 30 minutes on a side.  This took a few hours because the tank has two top areas and I wanted to be sure to get both of them.  Next I rinsed three times then dried the tank as before in preparation for sealing the tank a couple of weeks later.  
  I sealed the gas line openings before using the tank seal with ziploc bags with rubber bands (no leaks this time) and was sure when draining the excess sealer out to blow through all three small gas lines to make sure they weren't blocked by the sealer.  I did this after wiping any sealer off the gas lines.  I had put small insulated copper wire in the two smaller gas lines to keep them clear but that didn't seem to be necessary as the sealer flows freely plus I blew the gas lines out.  I actually used a quart and a half of standard tank sealer which in retrospect seems like it might have been overkill based on how much extra was drained out.  The POR 15 kit has just a quart.  
  This total process took me 4 days.
2 July 2012