Summarily, the passage is simply saying, if ones car is hit with eggs, it's best to wash it off immediately or risk re-doing the paint.
The sentences which highlight caution to the reader are:
The first sentence tried to quantify the amount of damage. The reader is able to evaluate the cost of damage eggs can do to a car;
The fourth and fifth sentences: urges parents not to encourage egg smashing on cars and goes on to highlight possible damages that can result from the same.
