How should I prep for a color change over existing good paint?

Sand the door or fender that is damaged down far enough to see what color primer or sealer is directly under the red paint. Replicate that when you start painting the replacements. If her car is still drivable, clean it up and take it to the auto paint store you will be using to buy the paint and get them to scan the existing paint so they can feed that info into their computer and make a better match to the older paint, instead of using the stock mix codes. It may still stick out like a sore thumb if you just paint those 2 panels without blending onto the hood and rear door or quarter a little ways. Get on You Tube and watch some of Paint Society's video's. He has several showing how to blend. If the replacement parts are completely dent free, wet sand them really well with 600 before sealing them with same color sealer that was on the original parts under the red. The wrong color sealer will change the color of the base coat. Good luck with it.

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