What to use to clear paint a few pieces of brass?
I believe that crystal clear enamel or brass lacquer (actually not a lacquer- it's polyurethane) may be what you are after (enamel connotes painting metal). Please pardon the product recommendation, but Rustoleum Crystal Clear Enamel works pretty good, but Mohawk Brass Lacquer is specifically for the job.

Spraying it on makes a physical difference- it is more durable... aerosol is closer to being a vapor deposition. The trick is to use 2 light coats. But I suppose that you could use any old brush-on polyurethane; for what it's worth, I think an oil based one might be a little more durable. 

Don't use shellac, as it is actually made for penetrating wood, so it won't work on metal (although what people call shellac and lacquer are often acrylics or polyurethanes). There are a variety of qualities regarding epoxy. Some epoxies will become cloudy or yellow with age, so if you don't want that, aim for something that is used more for finishing rather than sticking things together. Also, be aware that an epoxy might be more difficult to remove. Overall, I think epoxy might not be the best choice for you.