clean the area very well. let dry. spray on more undercoating. rust is caused by not cleaning your car very well. you can drive in the salt, but you need to clean all the surfaces very well to prevent rust. the main problem people have is that it is usually too cold in the winter time to wash their car.
if any rust is starting to form now, you cant do anything to prevent it. IT WILL COME BACK, unless you replace the affected areas with new parts. if no rust is forming, you will be ok.
undercoating does not prevent rust. it simply slows down the rock chips under your car. once the paint is chipped, rust has a better chance of forming. but, the most common rust starts in the seams and works its way out. it very rarely starts from the outside of a panel, unless the paint failed.
if any rust is starting to form now, you cant do anything to prevent it. IT WILL COME BACK, unless you replace the affected areas with new parts. if no rust is forming, you will be ok.
undercoating does not prevent rust. it simply slows down the rock chips under your car. once the paint is chipped, rust has a better chance of forming. but, the most common rust starts in the seams and works its way out. it very rarely starts from the outside of a panel, unless the paint failed.