Aluminum was my first choice, but it's so expensive now! I've looked at cedar, redwood, and treated wood. I don't want to use treated wood, because I'm concerned about the carcinogenic potential. Redwood is expensive, but I'm leaning in that direction. I'm concerned that the high humidity of a greenhouse will necessitate a resistant wood like redwood. What are your suggestions? Could I use an inexpensive wood and then paint it? If so, what kind of paint is safest?