I can't say that an undercoat is necessary, but it may be cheaper than your paint when doning several coats. For me I used a high-build primer because I was trying to help hide some drips/fiberglass strands where the deck joined the hull that I had not sanded all of the way down.

 Over 30 years ago I spent 2 yrs mainly in South & central Australia as a missionary. I remember seeing the henly on the Murry at Mildura. Where do you live?

Ralph

Iam currently building a sea island sport & are about to finish it .Iam going to paint my hull as I intend to fish for large pelagics out of it & many of these are very toothy critters so plenty of scratches are the order of the day After much consultation with paint manufacturers here in Aus I have decided on 2 coats of undercoat Iam also going to use single pack paint as it is easier to patch up & Iam told it is a little more impact resistant than 2 pack the secret to getting a good finish Iam told is in the preperation of the base coats sanding to 320 grit minium for a top finish also warming the paint a little above ambient temp also helps Iam told. Give NORGLASS a call Phone (02)97082200 here in Aus they are very helpful Also check out the A.K.F.F. (Australian kayak fishing forum) website they have lots of info  especially in the HOMEMADE section  Regards Wal