I've begun painting a polyurethane front lip for the S. However, i completely forgot to sand the urethane before spraying primer. Will this be a big problem? If so, what do i need to do before continuing with the base coat?

 

Yes, it will be a problem down the road. The primer will not adhere properly onto the lip. This will cause adhesion problems that will result in chipping,peeling and your paint job will not last or be durable. Your best bet is to strip/sand off your primer and start over again.