Here's a couple of notes from my experience. I use specialized coatings for my work and they are temp sensitive so I don't do outdoor work below 50 degree temps. I also have no experience with cold temp coatings as the previous poster suggested, but his recommendations may be a good choice. I can definitely say that painting in a warmer environment is the best choice if you can do it.
You mentioned that this would sit on a concrete platform. The ends of wood beams are like straws and suck up moisture which soon leads to rot. The areas where your wood may come into contact with concrete or be exposed to excessive moisture need to be protected or they will not last, especially since you're using untreated.
We've had good success with two products...Cedar-Shield, an emulsion of silicone and cedar tree oil...100% Non Toxic Wood Treatment Products Treated Wood Alternatives Deck Garden
and Elastolock, which is a rubberized coating that also makes a great primer. ElastoLock is from Daich Coatings Corporation
. This can sometimes be found locally in paint stores but it's from a Canadian company and they don't have real wide distribution in the US yet.
If you can't find it locally, contact me here or go to my website below and I'll arrange to get you some. We use both products when we build wood decks because we use untreated wood so our low maintenance coatings will adhere.
BTW, it's necessary that the Cedar-Shield drys for 5-7 days B4 you apply anything else. Drags things out a bit but it's definitely worth it. The easiest way to apply it B$ construction, is to put a tarp down big enough to hold all your wood and build a little dam around it with scrap wood so excess product doesn't run off. Then just use a cheap pump sprayer to liberally coat the wood, paying particular attention to the ends. We like to do it twice for good measure and we also reuse whatever doesn't get absorbed.
Hope this helps. Protecting your wood from moisture intrusion is the single most important thing you'll do to achieve a long lasting & low maintenance result.