With regards to installing rails and getting a good seal, I thought I would throw out a tip that I got from a yacht restorer who also does design.
We call this "bedding". The way I do it to make a really neat job of it, is to lightly trace the outline of the part where it's going to live on the boat. Remove the part then tape around the line, leaving about a 1/16" gap outside of this line. Use some denatured alcohol to remove the pencil line then apply a bead of your goo of choice. Place the part and lightly tighten it down (just enough to make the goo ooze out around the edges of the part, no more). Wipe up the ooze out and leave it alone for a few days, until the goo cures. Now remove the tape (if you haven't already) and tighten down the part good. This smashes the part into the cured "gasket" of now cured goo and makes a very good seal. The tape around the part thing, leaves a very neat and professional looking edge. This line can be used to paint against too. If you tighten down the part, before the goo cures, it can easily get squeezed out of the joint and leave you with a paper thin smear of goo, which doesn't realy seal much.
The above quote brought to you by Paul Riccelli - Riccelli Restorations - Eustis FL (I am not in any way affiliated with Paul, other than he has very patiently explained a lot of boat building technique to this newbie)