I have used two products for this very purpose in the past.

Duplicolor Clearcoat is available at any auto parts store for about $7 per can. It is a lacquer finish. Good news is that it applies easy, is relatively inexpensive, dries quickly and unused product can be saved for future projects. Bad news is that it will not hold up to gasoline or other solvents even after being fully cured. I learned this the hard way when all the tedious work that I had done to restore a license plate was ruined with a small gas spill.

Now I use Spray Max 2K Clearcoat. I have used this on three plates that I have restored and love it. It is a two part clearcoat that comes in a rattle can with a special mixing valve. Here is a link to the product on Eastwood--

http://www.eastwood.com/spray-max-2k-high-gloss-clearcoat-aerosol.html

I buy it at my local NAPA paint store for $20 per can. Expensive, but bullet proof. This gives you the same finish and durability of professional two part clear in a small quantity that does not require all the work to mix then clean your spray gun. The spray pattern that comes out of the can is similar to what you would get out of a gun, not a traditional rattle can.

Once you mix the two parts together, working time is limited. You can extend this by a couple of days by storing the mixed product in your refrigerator.

Good luck,

Maybellene