Have a look at Intergrain products. I used their two stage process on our outdoor table over 11 years ago and it is still pretty good. I've also just used it on the inside of an indoor planter box that we are putting into commission for what looks like the first time in 35 years. The first step is an anti fungal treatment that soaks into and helps protect the timber while the second stage is a sealant. They aren't particularly cheap.....from memory it was around $20 for a 1 litre tin 11 years ago.....but it goes a long way and you don't have to reapply for a long time. I'll see if I can get a happy snap of the tins and table tonight and post it.