What is the best way to re-waterproof older gore tex that leaks. I have an older Columbia parka that needs this done to.
It is either waterproof or isn't. Neither Gortex or Omni-tech fail very often waterproof wise. Either the membrane deteriorated, Seam failed or has a hole.
With that said You are most likely are talking about the DWR not working and the outer fabric absorbing water. That's very common.
Nikki wax has a cleaning and DWR restore kit. Granger has one also.
Both are great products.
Also normal detergent will kill your DWR