As with Saturday's thread, chances are your problem lies in surface contamination. 

https://www.clcboats.com/forum/clcforum/thread/46903.html

After sanding, vacuum carefully and then wipe down with denatured alcohol and a clean rag or paper towel. I use cotton painter's rags that have never been used before. (After use on the boat they get washed and go into the household rag collection.) Wipe down a second time with denatured alcohol. Apply epoxy after the alcohol has evaporated off.

first, I would review the link below:

https://www.clcboats.com/shoptips/epoxy_and_fiberglass/fiberglass_weave_epoxy.html  just to make sure we are all talking about the same thing and understand what we are trying to do here.

second, just to be clear, you apply varnish over sanded epoxy/glass,   so the criteria of when the epoxy/glass is ready for varnish is after you sand it that it is smooth (you can simulate what it will look like by wiping denatured alcohol on the surface....before it evaporates, is what your varnished surface will look like)

if you need to sand between layers of expoxy, vacume the surface with to pick up the majority of the dust, wipe with a clean/wipe paper towel soaked in denatured alcohol, let it dry/evaporate off.  don't touch the surface with anything else....it just gives the opportunity for contamination.

fwiw, after i roll on a coat of epoxy, i always go over it quickly/lightly with a foam brush to knock down any bubbles or drips or sags that may have developed on the surface.

most people will not have  'perfect' finish before varnishing.  but if it passes that simulation test i described above, consider yourself done.

most varnish jobs look better than they really are becuase people look at them from a couple paces back.

hope that helps

It seems that your 'glass cloth should be filled on the second or thrid coat of epoxy.  Sanding should be after the fill coats. 

The 80 grit is the one that takes out the hills, drips, drags, etc.  I usually start with 60 grit others differ. The lower grit should leave you with a smooth flat no shinny parts "scratched" surface. 

The 120 grit is to take out the uniform scratches from the 80 grit. If you are varnishing you might want to do a 220 grit to get rid of the 120 grit scratches. 

Then you varnish, no epoxy. 

   I sincerely thank each of you who have taken the time to share your experiences and knowledge.  You all are suggesting essentially the same aproach, which I will follow.  The mystery as to why the glass cloth wasn't filled by the fourth epoxy fill coat must be do to some form of contamination.  I'll attack it with another agressive sanding with an 80 grit, followed by a thorough cleaning and a denatured alcohol bath, and one more coat of epoxy.   If I like the results I'll sand again to prepare it for the varnish process.  

   When you are applying epoxy or varnish it is good to have a bright light shining on the work. Work your sight lines on the light from the light looking for thin areas. Keep the roller "wet". Remember it won't put layers on if "dry".  Sometimes I think a squeegee is better. 

   I've a similar issue on a test piece I'm making prior to glassing, epoxy, varnish on my deck. I've used CLC recommendation of Mohawk ultra penetrating stain cut with denatured alcohol to reduce color. Then glassed filled the weave and sanded after third coat.. fourth coat produced an uneven surface using a brush I got lines where it seems the epoxy ran away from the surface. Tried with a roller and got an orange peel look with spots that appeared to reject the epoxy( see pic focus on right side)

https://www.imagevenue.com/ME126I8J

I'm applying thin. Is it just a matter of a fifth coat now?

if it continues?????

���Personally I'm done after three coats and one good sanding with 60, 80, 120, 220. Certainly not all that sanding between coats. Five coats?......maybe the varnish. But not the epoxy.

Apply with a roller and tip out with a brush to ensure its even and get rid of any bubbles.

   I'm probably too late for the original poster, but after my first build (a Kaholo), I'l definitely be using a squeegee to apply the later coats of epoxy on the outside.  I just use a cheapo shower squeegee, then only sand drips and runs between coats (at most 24 hours between coats).   Then yes, the final sanding must get all the imperfections.  The squeegee cleans up with vinegar for subsequent uses, and I have plenty of rollers leftover for where I really neede them--the varnish.  

   you might find some help in these videos from a master boat builder...

https://www.offcenterharbor.com/videos/mastering-epoxy-russell-brown-part-11-rolling-tipping-interior-gloss-coat/