Generally the rule of thumb is that you need two coats of epoxy to encapsulate the wood. Even after sanding you probably have enough old resin deep in the wood, so any resin used with your reglassing would actually be the second coat. If you think your sanding went extremely deep then do a quick seal coat with a small amount of epoxy squeegeed over the surface before reglassing. I think one more coat of resin over the glass guarantees that the fabric is completely sealed. If this coat has some thickening material in it then you can flatten the surface quickly, too. Microballoons are on the expensive side of possible fillers. Thereare many options which cost less and there are other options which may not save you money but will add benefits.
6or example: powdered graphite not only thickens the resin so it fills the weave but it also strengthens the resin and makes the hull more slippery so it slides off of rocks better. Powdered aluminum makes the resin tougher. An "earth based" powdered pigment might serve as both a filler and an undercoat. You can even make your own! Rust has a nice brown color, charcoal or laser toner are black, and chalk or pulverized plaster would be white. You can get jars of other pigments at art supply stores.
If you skip the second coat of epoxy and go straight to paint you'll probably be OK, too. Let the boat sit a few days after glassing then wash it well to remove any amine film. Chalk is a common iten used in painting the base layers on canvas-covered canoes asit fills and flattens the texture of the canvas. It should do the same trick in smoothing out the weave of the fiberglass fabric.
While there are some special primers available to use on fiberglass before coating with boat enamel, some people skip them and use the boat enamel alone. Thicken the first coat, sand it smooth and put on the second coat straight from the can
PGJ