Hi Everyone:
           I am skinning an engine part and the first several coats went well, and I made sure to follow the rules about sanding down the high spots after a few coats of resin, scotchpading the low spots for good adhesion of the new layer, etc.  When I thought I was done, I was starting my polishing sands with a 400/800/1000,etc, but after the 400 grit, these weird white "worms" could be seen and spots all over.  I had not seen this before, but I had used my heat gun to remove bubbles, so I wouldn't think they are air bubbles.  Is this a bad mix on my end?  Maybe I didn't mix the resin/hardener enough?  BTW, I am using a 3:1 epoxy resin and hardener from US Composites. 
Any help in understanding potential causes so I don't repeat would be appreciated.

For now, I figure I will have to sand the blemishes down and do another few coats.  Bummer...

Thanks!

Looks like low spots or voids that have filled with debris from sanding? 

Doesn't look like a mixing error or anything like that to me, which is good.

hi.  i also had the similar situation when overlaying a hood.  after thinking and analysing,  here are what i think the problems:-

1-not using the correct resin. skinning resin is thick. i was using resin less than 500cps.

2- after one layer or resin, sanding is required before laying another. thin resin layer will become too thin after sanding. what you are seeing actually is the edge of resin,.peeling underneath new resin layer.  again, not using correct resin. 

3-not mixing more than 3 minutes. the part of the uncured resin also causing this white worms. they are delaminating, absorbing moisture if wet sanded and other stuff.  ensure to mix the resin thoroughly and please change to new cup before using the mixed resin. 

4-moisture.doing it outside during raining time? bad idea.

5-surface contamination.ensure the brush is clean, and the previous resin layer ia free from amine blush or water from wetsanding. let them dry first.

and until now, my hood is still having a resin peeling. if your resin too thin, maybe a several chemically bond resin layer is needed. make sure to lay before amine blush is forming. 
i am still not done any experiment feom my theory. but i know they are peeling resin.