I don't think it is standard practice to sand between fill coats. Fill coats are usually done on fairly green epoxy. Sand green epoxy and you will have a number of problems that will be showing up as you work...not the least of which is the white specks. Most fill coats are put on green epoxy because of what some people call the "chemical" bond. I believe the chemical bond is not as critical as is commonly believed. Surface preparation is the most critical component in good bond. Green epoxy does simplify preparation and helps avoid amine blush. However...it is best to use a blushless epoxy and treat it as one that will blush. Anyway...between fill coats I scrape where needed with a carbide scraper (the type of scraper is shown on my site under one of the albums. Not much there yet...but maybe someday). After scraping I recoat. I let cure completely...usually at least a week, preferably more...before doing any sanding.