Will charred polyurethane cedar deter termites?
I doubt it and even if it did, you would have to basically make charcoal out of it to make sure there is nothing &quot;edible&quot; left.
The gold-standard for termite protection is to break ground contact.  I'll recommend a concrete or brick &quot;foundation&quot; for your storage rack which could then transition to wood for aesthetics.  If you are hyper-sensitive about your wood, use termite guards on top of the masonry before you transition to wood.  Termite guards are just steel, usually galvanized, and serve to further make the path from the ground (where the termites live) to the wood (where they eat) more tortuous.