If I understand you right then you want to bond the underside of your deck to the tops of the glassed stringers? If so then you want to use Cabosil. It's a fine powder silicate that is dangerous to breath so wear a mask. Mix it with the resin first then add the catalyst. If you are using epoxy then add the hardner after you have mixed it to the desired thickness with the resin. YOu will have to make it really thick before adding the hardner. Once you add the hardner or catalyst it kicks off really fast in the pot so you will have to work fast to get it out of the pot and onto the stringers. Hope this helps some.
strick