Getting a glass like finish on sheetmetal

You want to either preserve bare metal with an etch solution/rust converter such as picklex or prime it as soon as possible after sandblasting to prevent rerusting. The picklex and similar products are used to prevent rust if you will not be primering right away. These are good products if you are stripping parts and wish to preserve them for paint all at once AKA paint day. I prefer this way of doing it, less cleanup and waste of paint trying to guess how much you need to cover everything. Also keeps you in the time window for chemical bond to your topcoat. Body work throws a wrench in time windows, it must be done first.

I find urethane to be an easy paint to spray smoothly. The proper mix ratio and thinner for the temperature you will be working at, Good gun technique, (no double passing) and waiting the proper time between coats are all important basics needed to get a smooth finish. Technique takes practice, but the basics are spraying at the right distance from the surface for the gun and paint you will be using and keeping the gun straight to the surface, try not to spray on an angle. Sometimes difficult to do with irregular shapes and small spaces. Your hand becomes accustomed to the weight of the gun and hose drag with practice. Just like using your computer mouse. Hope you are using proper breathing protection from iso's with that urethane.