I use Window frame sealant external white that builders and window fitters use, obviously you will also need an applicatin gun for the tube of sealant. If you do use either don't cut the application tube to get a thicker bead out it is messy. Use it as it is and build up layers it ends up a neater job.
Or you could go to your nearest caravan spares outlet see what they have to offer which would be long lasting..
Bob
May 2009