Hardware store people generally don't know too much but like to refer to what they have. Fibreglass resins will breakdown if exposed to uv light over time, and this includes epoxy resins - you must coat it with a protective barrier paint; they are also very messy to use as a sealant in the sort of situation you have; additionally fibreglass does not bond well to any sort of plastic (like your polycarbonate) to get abond to the dome you might need to 'key' it by grinding back to the glass surface (assuming its a fibreglass dome - if plastic forget it). Personally I would not put on anything that cannot be removed if necessary. There are many long lasting butyl tape solutions available used to repair caravan rroof leaks that would work well and be far less mess that fibreglass. I used a wide butyl tape (white in colour) to waterproof a hinge seal on my camper and it works great.
Have a look at StickNBond products:
http://www.sticknbond.com/
That is the stuff I brought in for my camper sealing job - mainly because I wanted a white tape that would match the camper roof and last under our UV for many years.