I use this stuff, available at O'Reillys auto parts stores and other places. It doesn't require a catalyst. It won't change the surface colors much at all, usually things pop a little more after clear coating with this and it tends to even out colors some. I just shot some last night actually, dries quickly as well, very easy to use.
You do need a spray gun of some kind to apply. Surface needs to be clean of course. I'm not sure how well it would hold up outside but probably be OK. Can always pick up a $20 gun from harbor freight if you need one, a quart can is about $30.
As far as sanding the surface before clearing, you can. I find it easier to abrade the surface with a scotch-brite pad. Of more importance really is that the surface is clean. I use "wax and grease" remover which is also available at any paint place. There have been times when it's been completely impossible to sand the surface because of the shape, and it's still been fine.