This happens on occasion. You should try to avoid it, but it happens. This is precisely why we build our boards with a clear coat "unpainted" finish... so that we can avoid the problem boards with painted rails are plagued by, which is after some use the paint on the rails tends to chip and the boards get really ugly. Seriously.
You can dent and ding our rails up, but they don't chip. If you get black marks, those can be sanded off as our boards feature a sanded finish, but be careful not to sand thru so do it sparingly.
If you're overly concerned with dinging the rails, you can buy a product called "rail tape" that will offer extra protection.
What is the best paddling technique?
How do I paddle my SUP?
Does the paddle float?
Can the paddle be used as a kayak paddle?
Will Tower Made cut and glue my paddle?
How do I know what length my paddle should be?
How do I cut a SUP paddle?
How do I epoxy my paddle handle?
How do I care for my SUP paddle?
Can my paddle damage the side rails of my SUP