How numerous / pervasive are these epoxy powder-filled holes?

Have you tried simply using a vacuum cleaner to suck the dust out?

I'd be very reluctant to use compressed air from a typical air compressor to blow dust out, in that most compressors used in home shops potentially can blow oil out with the air they compress. (There are such things as oil- and moisture-traps that can be added to the air output line but they're not common unless the compressed air is being used to spray finishes.)

If you're planning to paint you needn't worry. As long as you do a proper job with the subsequent coats of epoxy, paint hides a multitude of visual defects we notice in working close-up.

If you want to varnish your epoxy, it's possible that the next coats of epoxy you apply will soak into the powdered stuff in those holes, making them less apparent. What might work better would be to use the fine tip of a super glue dispenser bottle to drip a bit of cyanoacrylate glue into those dust-traps. That stuff being vastly more 'liquidy' than epoxy, ought to readily soak into the dust and making it virtually disappear. CA's commonly used for 'tacking' panels together, ought to present no problems with subsequent coats of epoxy bonding properly.

If all else fails bear in mind that once you're done you'll be looking at your watercraft from farther away 99.9% of the time you're using it. Those white specks out to disappear once you step back for a wider view. 

Hi Cloyd,

oh pinholes.....

first, hindsight is 2020...but let me share with you why pinholes like this probably happened.....

when you put the first coat of epoxy on any wood, it is critical to ensure the temperature of the wood/air is decreasing.  the reason is wood breathes....as it get hotter it exhales and when it gets colder it inhales.  so if you are putting that first coat of epoxy on when it's getting hotter, little champaign size bubbles (called outgasing) can be blown into the epoxy and then when you sand, you can break open these little bubbles and then they fill with epoxy dust and become white.  and they are a real hassle to clean up.   

fwiw, once the first coat of epoxy is down, the wood is now sealed and subsequent coats are not suseptable to outgasing.   so after the school of hard knocks, i am very sensitive to when i apply that first coat of epoxy to any wood surface.

on cleaning them up,  there are two techniques i have used that work reasonably well.  one is clean water under pressure and a brush (i use a toothbrush).  the other is denatured alchohol and a brush.   denatured alcohol and a brush is safer and easier to then get ready for another coat of epoxy.  water you have to wait longer to ensure it is totally dry.   the next coat of epoxy put down after you have cleaned up bubbles should be using epoxy that is warm so it is as thin/liquidy as possible and will run into and fill your little divots.

i hope this helps

h