Leaking metal rain tank

I have a few tank problems. One is easy, I just need to replace a tap.

The one that is being a headache, has what appear to be multiple small leaks across the area where there is a diamond shaped piece of metal that the tap in the water tank comes out of at the bottom of the tank (a common enough area for leaks to appear). .

This tank HAS been repaired before. However when the last owner inherited the house it from his dad, he didn't really live there full time (just as a weekender) so when he had some tanks die, he did not repair them. As long as he had at least one 18,000+ litre tank (and the very slightly saline bore) he didn't really care.

We (on the other hand) want our water back.

So, I went ahead and got rid of the old sealant, leaving the tank disconnected from the others. I had already used a small amount of bore water to see where the leaks were so I was well prepared. I also took a very mild brass metal brush and went over the surface a little bit, just to be certain that ALL the old sealant was gone. Then I went to the hardware store (out here in "rain water only" land) and told them the condition of the tank and asked which sealant to use. They then sold me a clear silicone sealant: flexi-seal pro-seal, industrial grade, clear

I got home and cleaned the area well with meth-spirits to be sure there were no oils or other residue there. Then I let the February heat take off all the spirits for a whole day. The next day, mid-morning when the tank was dry and warm, I went out and VERY cautiously and slowly started to apply the sealant from the caulking gun, smoothing firmly it into every last variation of the surface by using a disposable paint brush for maximum penetration into every little surface variation. I wanted to be very sure that what I put onto the tank would stay there and match the metal perfectly.

After the first layer of sealant was there and as perfect as possible, I waited about 6 hours and added more. The next day I added more again in the late afternoon. After all my layers of sealant were good enough (in my estimation) I had a very nice looking even seal, and I THEN waited a week for the whole patch job to dry safely in the heat for several days.

A week later I chanced a test, by putting in a meter and a half of bore water (the tank is 3 meters across) . NOT ONE DROP LEAKED!!!. :U This seemed great, so I added a bit more water to see how it would deal with more pressure, and still not a leak. :U I left the water there for SEVERAL weeks (it nerver lost a drop) before I drained out the bore water and hooked that tank into the rest of the water tank system and transfered in soime rain water.

NOT A LEAK... UNTIL - Our first rain on the 23rd of April

Then yesterday we finally got our very first rain. I chose to have a look before it got too dark outside (if there had ben a problem), and I saw a problem.

:o

Water had started to well up and forward along the tap pipe that comes out of the tank. I felt arounjd and it was obvious there was pressures INSIDE of the sealant. I have no real choice with night falling, and I had just suffered a VERY BAD gall-bladder attack so BIG or heavy work was out.

I grabbed the sealant and squeezed and it stopped leaking.

So..as a temporary measure I have taken a rope and wrapped the pipe for the rain-water tank tap, as if it were a very tight spool of plastic rope. It is not leaking anymore, but I KNOW this is not a real solution. I need to find out what kind of sealant to REALLY USE, and how to do it, when I fix this thing FOR REAL.

I would love serious input from the experienced. I know that I have GOT to get this project/repair on the road before it is wet out all the time, or there will be a lot of limits on what kind of repairs can be done.

CHEERS

By the way, I am disabled, so there are some limits on what I can do (lifting and such) these days, but I am decent with my tools, so I am not afraid to try something new.