If your Mustang isn't going to be a concours trailered or thoroughbred and you don't want to go through all the trouble of chemicals and such, and you like driving your Mustang like I do, you can use Eastwoods Detail Gray paint that comes in an aerosol 13 oz can, item #10036Z. This paint will give you a natural looking finish.
Also, check out all the other paints and restoration tools/equipment Eastwood has to offer.
I used Eastwood paints and products over a decade ago on my Mustangs with no troubles at all, and I drive mine and once in a blue moon get stuck in rain, but don't have to worry about surface rust since the parts have been painted under the hood and the undercarriage. But, I do make an effort to wipe away any moisture when I get home or back to the show field.
By the way, I show my Mustang in concours "Driven" and I'm 2 points away from applying for my grill medallion and placing our Mustang in the conservator class.....Hope this helps in your effort.