moisture-proof barrier, recommendations?

[CabinetSage]

Hello, I am planning to build planters since the season is getting rife now to grow!

However, these planters will be outside and potentially will get wet, I was planning to make them out of pallets that I find somewhere.

What would be a budget-friendly, or most cost-effective way for me to seal the wood and prevent it from rotting?

 

[Katomi woodsmith]

I made mine out of scrap wood, I varnished the outside but left the inside untreated as I was growing food to eat.
The liner/membrane needs to allow water to drain otherwise your plants could rot.
I made extra slats for my planters so that I can replace as needed
Ant to be honest, as mine were scrap wood, if they rot over time so be it. My bought and expensive planters are rotting faster than my home made ones
Advice when you decide on the height make them so that you can sit and work on them, future proofing