[Foot Stomper]

One of the things that's been mentioned here is going to be very helpful if you consider it.

If you clear the tailgate after painting it your main color, you can apply white over the letters if you scuff first. Then any mistakes, changes, challenges you have are simply on top of the clear. This process protects your main color as long as you don't burn through the clear. Once you have your letters to your liking, scuff the entire gate and clear everything.

This process is used widely by airbrush artists in order to "lock down" or otherwise save work already done.

Good luck with your tailgate!