Welcome to the Bunnings Workshop community. Thank you so much for joining us and sharing your question about painting your timber support structure under your deck.
Before I offer any recommendations, would it be possible for you to post a photo of the supports you are planning to paint? This will give our members a better idea of what you're painting and will then let us offer you an ideal solution. To start you off, there are a few factors to be aware of concerning your decking supports. Is it still oiled? If it is, that oil must be either washed or sanded away. What kind of paint were you planning to use? Will you be oiling it again or will you be using actual exterior paint? How old is the decking structure?
Once we have more information about your deck, we'll be able to recommend products to use and suggest steps to take to paint your support structure.