I am having the exterior of my house painted in a couple of weeks and before they get here there is some trim and some wood siding I want to replace. My question is, do I need to prime both sides of the trim/siding before I put it up to protect the wood from the weather? Or can I just put it up and then prime what is showing so he can paint it? FWIW, I live in Southern California, so the weather isn't very severe, if that matters. Thanks in advance for any opinions.