You have two real choices to seal the holes back up in your deck. First, if you are going to put in another seat or leaning post, you could always countersink the holes and put some flathead/ovalhead screws back into the holes with fresh 5200. Second option, counter sink the holes (NOT alot, just enough to get clean fiberglass. This will also remove the 5200 in the hole so the gelcoat/marine tex will have something to bond to), mix up some marine tex or gel coat and fill the holes. If you use gel coat, leave enough gel when you fill the holes so it is raised above the deck. When the gel dries it will shrink back some otherwise you will have too do it 2-3 times to get the same level as the deck. Also, you can us acetone to remove the old 5200 by soaking a rag with it and lay it on the 5200. Check fairly quickly to see it soften then scrape with fingernail (depending on how much you have to remove). BECAREFUL, you have rough glassed in deck (meaning that it does not have a shinny surface like the hull.), if you let the rag with the acetone lay to much, it will start to break down the gel coat on the deck, then you willl have a much bigger problem. Good luck!
Weedy