I own a media blasting business. We use aluminum oxide in place of sand for rust removal. It is less toxic and you can recycle it many times. Sand shatters into dust after one or two uses and of course it is toxic to the lungs (silicosis). Any media that can remove rust can warp sheet metal. There is a big misconception that heat from sand blasting causes warping. It will get hotter sitting in the sun. It is actually caused by suface peening which expands the metal. It is fine to use it on areas that are ribbed or curved or edges, such as floor pans, jambs, engine compartments, heavy metal,(and bettery trays), as this type of metal has internal support against warping. Never on flat sheet metal. We would blast the back of your bumper and your battery tray for about $30. We'll worth not dealing with the mess and aggrevasion and wear and tear on your compressor. So call a local sandblaster for a quote. As for sand, what ever you use must be super dry or it won't flow in your sandblaster.