When it comes to seam sealers there are a few options, there's 2 part seam sealer that takes a special gun or there's the brushable kind that comes in a can. The best would be the 2 part but the gun for it is pretty expensive, I don't think the brushable kind would hold up well in that application. Your best bet might be to use a 2 part epoxy, it will work the same as the 2 part seam sealer and hold up much better than the brushable stuff. If it were mine I would scuff the area good and then run masking tape along both sides about a 1/4 inch from the seam and then brush the epoxy on with an acid brush, just make sure to peel the tape the second your done or you won't be able to get it off.
http://m.homedepot.com/p/Loctite-0-8...5391/100371815