The paint can’t adhere to the surface because of contamination by oil, grease, wax or polish. So the paint draws back, leaving unpainted areas, usually in the form of small spots. To remedy the situation, if the paint is still wet, wash it all off, allow the surface to dry and thoroughly harden, then rub it down using wet and dry abrasive paper (or waterproof silicon carbide) and warm water with a bit of detergent. Rinse the surface thoroughly and allow it to dry before repainting.