Hi Ned, 

no sanding is needed between fill coats if you are recoating in relatively quick succession.

when i do fill coats, i usually am doing that within 24 hours.   if i was waiting a week or more, i would probably hit it with a light sanding/scotch brite pad.  but less than that i would not bother.

that said, if you have some bumps or drips, leveling it out sooner rather than later will save you some epoxy/sanding if you are concerned about that. 

the most important thing to keep in mind in fill coats, in my opinion, is thin coats that are focused on filling the weave....and not overly thick applications that creates a lot of drips and sags.   this will save you epoxy and reduce sanding time as laszlo points out in the posting entitled "Not enough epoxy with the kit".

h

   Thanks for the information and the quick response, I did the fiberglassing yesterday and unfortunetly wont have time to do the remaing coats until this Saturday, I think I'll sand down the couple sags and hit the rest with  a real light scotchbrite before doign the full coats.