Despite all my efforts I have not been able to get a pinhole free surface. The materials are great, and the closest I have come, but to date I have been unable to figure it out. 

I have also had this problem.

I do a lot of marine parts and in general it seems that the bond between the epoxy resin systems and polyurethane paints is not so good. I spoke with one of the NZ tube manufacturers about this and they simply said to use a quality brand and put it on thick.

I have also done some yacht masts with an adhesion promoter. It seems that this adhesion promoter is more like a thinner and I think is really intended for plastics like car bumpers so I am uncertain if this has worked or not.

One part on my own boat is holding up really well after two years of sun and salt. This part was epoxy coated, sanded and sprayed with a good two part clear. This leads me to wonder if the problem is more the adhesion paints to a sanded fibre and epoxy resin surface as opposed to one that is all epoxy resin as with a coated part.

But then I also hate coating parts with epoxy systems, they generally seem very sensitive, run/drain away, often fisheye as well being really hard to sand so lately I am now working more with coating epoxy parts with a polyester clear, sanding and then spraying 2 part clears.