We live in the south with fairly moderate winters, but we have more rain than snow. We water proofed our deck (which is a screened deck) on inside and now several years later the interior still looks OK and still has product but the edges for a couple of boards, is worn off and getting mildew etc. where the rain does come in on the deck.Is it possible to water proof around the edge without redoing the whole deck? Also if I do have to redo the whole porch do it do it over the old product (probably Thompson's or something similar) or do I have to remove it and then redo the waterproof?

Yes, it's fine just waterproofing around the outside and anywhere else you want to spot treat. You can bleach mildew if you want first, and let it dry.