Do you use the A-arms as jacking or stand points? Unless something contacted your A-arms, there should not be surface coating missing.

I use a rubber pad on my jack and use the painted frame to lift the car. I also take my exhaust off the car and wax the underside every year.

I have 40K miles on mine and I religiously treat the underside like the body. I drop the inner fender skirts and clean out all the rocks and road grime each year. I pull my suspension and clean the car and then retorque everything for the next season.

I know it's crazy, but I'm giving this thing to my now 2yr old when he gets his license.