I once knew told me that anytime he got something on the wood floor of his trailer he would use coffee. He told me to buy a pound of cheep coffee and sprinkle it on the wood and it would absorb almost any smell up to and including rotten fish. I don't know about diesel but I would clean it with orange oil cleaner, wash it down, let it dry and then sprinkle coffee on it and let it sit. When your ready to put the tank back, vacuum it up and install the tank. I don't think I would rush in and paint it. Paint sticking to diesel soaked wood seems to me to be a long shot. Good luck.