Barry
I would suggest a thinned coat first, to allow the polyurethane to soak into the wood and set up hard. Don't use the Quick-Dry acrylic stuff as it's not waterproof and it doesn't thin with oil-based spirits. Thin about 60%/40% varnish/white spirit and allow to dry for at least 48 hours. When you sand it again with 600 it will take on a silky glow (almost poetry, innit?). Then you can apply as many coats as you think necessary, rubbing down with 800/1000/1200 Wet and Dry as you go. I never apply varnish without thinning it first - even when brushing.
Works for me, but try it out on a scrap piece of veneer first if you're at all uncertain.
FLJ