Shed Talk - protecting exterior plywood - best way.

[DippedHeadlights]

I'm in the process of building a handy shed/garage extension for car parts. Timber framework and using 12mm exterior ply for the walls. Not a lot of money and gives a nice inside finish.
The outside finish doesn't really matter as it's hidden by fencing so no need for fancy dancy shiplap etc, just needs to be waterproof and not rot.
I think it needs something on the ply, not paint, some sort of stain or sealer ?
In the past I've tried a few garden products without much success, works well on cut wood but not on smooth, seems to flake off and need doing again within a year.
Any product ideas / suggestion as to what I need ?

DH