David the first thing I would do is toss the tack cloths, they should only be used once you have your first coat of varnish on.  More then likely the white dots are waxes from the tack cloth that have been driven into the epoxy by sanding.  The best way to get rid of them is a scour pad and hot soapy water.

The best wipe down while working on epoxy is a clean rag with denatured alcohol. 

David,

If the white dots appeared before using the tack cloth then they are epoxy dust in small pinholes in the epoxy. A bit more careful sanding will probably get rid of them. If not, or if you're concerned about sanding into the cloth, you can apply a very thin coat of epoxy with a foam roller before a final sanding. Even if there's a bit of dust left in a hole the epoxy will make it go clear again. And varnish will have the same effect on small amounts of epoxy dust. And absolutely get rid of the tack cloths. A clean cloth dampened with water or alcohol will do a great job and leave no residue.

George

Try blowing them out with compressed air. However, they do seem to dissappear with subsequent coats of epoxy.

Barry

The same thing happened to me.  It was dust getting into the tiny pits in the epoxy.  I continued to sand until I started to see the weave of the glass (not into the glass) and most of the small pits were gone.  There were still some that I just could not eliminate without sanding into the glass, so as George said, I added a thin coat of epoxy and that seems to have done the trick.  That was on my hull which I plan to paint, so I'll also be coating with Pre-Kote primer and then Brightsides paint.  I imagine, I will also have a few of those spots on my deck, which I do plan to varnish.  But after a lot of hull sanding, I'm a lot more familiar with the process.

thanks all, the white dots did appear before the tack cloths came and some did stay with the next coat of epoxy.  I c-a-r-e-f-u-l-l-y sanded some more and then hit them with the vacuum and water.... problem solved (at least enough that I can live with.  90% of them were right around the copper inlay on the transom :(

OK, time for varnish coat #2 on the deck!  Should be hitting the water early next week.  Will submit pictures.