Rob, by "automotive grade" they probably mean the Matthews Paint system which is a catalyzed urethane based, formulated specifically for the sign industry, they are owned by PPG which is a leading auto pain manufacturer and had all of their R&D behind it. If a good enamel lasts 4-5 years, Matthews will last 7-8, not cheap but we've been using it for decades on all of our signs with great success.
I don't have much experience using spar vanish because we use the Matthews clear or equivalent which is formulated specifically to use over the Matthews. Obviously the traditional Marine Spar Varnish will work fine over wood since it has been used successfully in the marine industry. From what I know about it though, the traditional solvent based finishes are getting increasingly difficult to find as they are being phased out by VOC laws, not sure how the newer replacements will hold up.
As for mounting, a good quality RTV Silicone has always worked well for me as it had good adhesion and allows for some expansion and contraction.