Yes , you can apply TK44 on some lightly rusted spots. The coating will seal the rust from any  moisture and will stop it from spreading. The coating stays tacky but will harden enough ( after 2 months) that it will not permanently pick up sand. Even if a bit of sand would adhere, the next rain shower will wash it off easily. If you want  your vehicle to be 100% sand free, I can recommend TC/UC , a polymer based undercoating that will harden completely like a plastic. Because this is a water based product, we recommend a light coat of primer over the rusted areas.