I had my first experience today of having to connect earth rods together to get deep enough to hit wet ground and get a decent reading.

It was at the top of a hill, 30cm wet soil, then 1m + of bone dry something and then presumably at some point some wet ground as the second rod length gave a decent result. Previous attempt with 2 rods  separated gave me about 350 ohms.

I was wondering how reliable the screw joint between the rods will be over time. Also should I have protected the joint with petroleum jelly or something, would it make any difference 1.4m underground and hopefully at least damp.

It probably doesn't make any difference but going by the effort requited and geology of the area I suspect I was driving the rod through Portland stone, a form of lime stone.