2008 Mustang GT. The roof has developed several rust spots, 4" in diameter, on the roof. Painting is not a priority right now. Any suggestions? I was thinking naval jelly?

 

Silicone grease maybe? Probably will resist washing away in the rain better. Or, just sand it out with an orbital, degrease, hit with a self-etching primer followed by a rattle can colour match. The latter will look better and probably better keep it from rusting more until you paint entire roof/car.