Yes, if the existing coat is sanded/scuffed then it should stick just fine. This is also the method for topcoating if you are not using the etch primer. It won't bond chemically like it would if the coats were applied at the right time though. I've had to topcoat cured POR-15 before and then later found that it will scrape off with a sharp blade in contrast to the original coat that requires a grinding disk to remove.