Late to the party, but SEM etch primer, or an alternative brand, is a great way to protect bare metal. It contains a mild acid so protect yourself accordingly. Applied lightly it will keep your metal clean as you work, doesn't gum up like epoxy when you need to sand or grind away, a light tooth and you can apply it over bare repairs and sanding blow through. It's non-sanding for 24hrs which is very handy for those last minute covers and prep work. Available in aerosol cans for sure, perhpas qt cans for a gun too. Again, contains acid so PROTECTION AND VENTILATION IS IMPORTANT.