Ive made three boats by the "stitch and glue" method. Mirror, a kayak and my tender. I have never used a primer or seen any suggestion anywhere of using one.
just used ordinary polyester resin and glass tape.
My tender has spent nearly every summer upside down beside the yard pool since 1978 without showing any problems with the seams. It is re-painted each winter though.
If I were to make another I might consider using epoxy rather than polyester but based on experience no real reason to do so.