Hi,
I would try fine wet/dry used gently wet with a drop of fairy liquid in it. Use a sanding block at all times, or, get hold of some polymorph ( here:
http://www.maplin.co.uk/Module.aspx?ModuleNo=35511&C=Froogle&U=35511&T=Module ) and use it to shape a sanding block to the deck profile.
Go gently until you have a uniform matt surface, then start again with varnish, beginning with 50/50 varnish/white spirit for first coat, then 75/25 for second coat then neat varnish.
Sand down with 1200 or finer wet/dry as above between each coat after allowing plenty of time to harden. You may need 2 or 3 coats of neat varnish to get a good high gloss finish.
Alternately, try a scraper, maybe a razor blade?
Ian