I park my 4Runner in the shade so the mildew buildup behind the gas door. What would be good to clean mildew without damaging the paint. I believe anything bleach-based may harm the paint. What's the alternative?
plain water and bleach.
Mix 1 cup bleach to 1 gallon water (I don't measure. add more if mildew is bad but no more than 50/50).
pump sprayer works, or bucket and sponge, or if you have a hose attachment, etc...
-pre-wet car.
-apply mixture and let sit for 15min.
-rinse with plain water.