I have a rain water cistern, and the inlet and overflow pipes (3" PVC) penetrate the wall at the top of the 7' deep cistern. The pipe penetration was done by placing an ordinary 3" nominal PVC coupler in the poured concrete wall when it was poured. The PVC pipes were then passed through the coupler when the concrete was cured and the forms removed. The pipes were not bonded with PVC cement to the coupler, they were just caulked from the outside. This failed after a year or so (now 5 years on) and the leaking water between pipe and coupler (through the caulk joint) made mildew/mold stains on the nice outside wall. Recently I picked out the old caulk and sealed it with 3M5200 when the joint was dry (water lower in the cistern). I did a good job of that. Now we just got a lot of rain and both places are seeping again - much less than before, but still some leakage down the outside wall. I think there must be another leak path between the concrete and the OD of the coupler. Builder says he didn't treat it with anything special. Wall is 10" thick. My proposed solution is to wait for joint to day again, then wick in some really thin (watery) liquid that will cure, and I'm asking the forum what to use. I know cyanoacrylate adhesive (superglue) comes in very low viscosity formulations (McMaster), and there are some very low viscosity epoxies used for electrical coil impregnation (not researched this carefully yet). Does anyone know or have experience with what will work best?