What is the best preserver for a plywood gate?

This really depends on what you're going to be finishing it with. If you're going to be using a wood paint or a wood oil, you're better off using Barrettine Premier Universal Preserver or Sadolin Quick Drying Wood Preserver which are compatible with most types of top coats.

If you want an all-in-one product which colours, seals and protects, then products such as Barrettine Premier Wood Preserver, Ronseal Total Wood Preservative or Cuprinol Ultimate Garden Wood Preserver will do all of the above. Alternatively, see our wood preservative treatments page for more information.