If your system is cryogenic you can absolutely still use wax and wax-like products. I sealed an aluminum can for something like 30 LHe cool downs with a mixture

If your system is cryogenic you can absolutely still use wax and wax-like products. I sealed an aluminum can for something like 30 LHe cool downs with a mixture of glycerine and soap. Future cryogenicists: glycerine+soap is amazing because you can wash it away with hot water before you finally take the part to get re-welded. Most epoxies/sealants can't be cleaned so easily, fouling any future welds.