It always amazes me, the sheer number of people who don't read the original message. Karl said he has the material, he does not want to go buy different material. He did not directly ask, but would seem interested in product(s) that could be applied to extend the life of what he does have, so those recommendations are on topic.
Karl, if you can prop your framework up on some cement blocks or similar to keep them out of the dirt and allow them to dry between rains, it will help a lot. Use enough blocks so things don't sag between bearing points. Paint will help, but requires recoating and maintenance. Coating with emulsified tar isn't bad.