Hello Sandy:
After only a year, your painting should not be appreciably dirtier than when you painted it, unless it has been sitting horizontally with the surface face up for that year. My suggestion is to gently wipe the surface with a clean dry lint free cloth.
I have varnished paintings with both Damar and Gamvar. The real thing I have noticed with both of them is that they require the room, the varnish, and the painting to be fairly warm - around 80 degrees is best. If the surface of the painting is too cold, then the varnish will actually bead up.
I use a spare bathroom for this purpose. I put the painting and the closed bottle of varnish in the room with a space heater for about an hour before the varnishing event. That way, both room and painting come to the same warm temperature point. I use a brush to apply. The varnish will be touch dry in under two hours.
As to varnish types, I like Gamblin's Gamvar. It is a synthetic varnish, optically clear. I believe that over time it will not yellow like Damar does. Further, it is easier to remove than old Damar - you can use milder solvents.
Hope that helps!
