I am planning to reseal my stamped concrete which is almost two years old now. Two questions:

1. How do I know if I need to reseal concrete. I see some white spots emerging, not fading, but tiny spots that feel rough on touch.

2. If I reseal it, what type of sealer I need to use. Water based or solvent based? How do I find out which type of sealer was used?

Thanks for help!!