if you are using MAS epoxy, it is non blushing, and while there is a lot of discussion about sanding and mechanical vs chemical bonds, this is not a particular issue for a fill coat and more of a vestige of older formulations of epoxy that 'blushed'.    as long as your surface is clean and has no contaminants, a new coat will actually adhere just fine with no further surface prep.

that said, i would lightly sand it with 100 grit paper....a higher number will work too..you are just trying to scuff it up a bit,  the fact you may not get into the dimples is not particularly important for this step.  if you happen to have a scotch brite pad, i would use whatever color/grit you happen to have....again you are just trying to scuf it up a bit. 

the most important thing, as mentioned above, is that the surface is clean and free of contaminants before the next coat of epoxy goes down.    i use a shop vac first with a brush to get all the loose dust off and then use a clean white paper towel soaked with denatured alcohol to finish off the cleaning...

don't stress out on this step.  its easy.

fwiw, i am not trying to dismiss the difference between chemical and mechnical bonds and the importance of surface prep....but this is a fill coat, not a structural bond under stress (like creating a spar) that would test the quality of mechanical bonding in a way that you would notice the difference between a sanded surface vs a non sanded surface assuming the underlying surface is simply clean and has no contaminants.

h

 I absolutely agree with hsira's comments above.  I will add that I don't see any reason to do fill coats on the inside of a kayak.  Fill coats are added so that you can sand the surface to a mirror smooth finish.  Since most of the interior of the boat (inside of hull and underside of deck) will be hidden once the boat is assembled, why expend the time, expense and weight that the fill coats would add to the build.  I also find that the interior fiberglass minus fill coats makes a nice sort of non-skid finish in the cockpit area.

i wanted to echo Marks comments on filling the weave on the inside.  i do two coat on the inside but don't try to fill the weave.  the second coat is to address any pinholes that may developed in the initial wet out and to ensure waterproofness.

the one exception on the inside for me is the area when the foot heel is going to be rubbing on the boat.  in marks picture above, it looks like he has some material (the black pads) to address the added abrasion that occurs in this area.  for me, i do something similar with a slightly thickened cabosil epoxy coat that i put down in two rectangles that correspond to where Mark's picture shows the black pads.   mark, what are your black pads made of?

h

On my last couple of boats, I have used 4" wide KeelEazy strips to prevent wear where my heals rest.  I also use the 2" strips to protect the side of the boats from paddle strikes.  It is a good product that can easily be removed with a heat gun.