New treehouse, cant stand to leave it bare over the winter

Hello- I am working on a treehouse, with Douglas Fir T&G flooring. I won't get the roof and walls done until next summer, so want to protect it over the snowy winter (Eastern Washington, in the hills).

I can't stand the idea of leaving it out in the weather all winter- The 18ga brads I put the decking down with are already starting to color the wood, just from the light rain.

Would it be OK to give it a quick coat with TWP1500 on the new wood, considering that next summer it will be enclosed with a roof and walls?

Since I used non-treated wood for the joists, and Doglas Fir Glue-lams for the beams, can I spray a coat underneath too?

How long would it have to dry before I could put a tarp on it for the winter?

Thank you very much-

Zeph