Use a filler
I'm no expert, but I believe you use a product called a filler, probably a high density filler. The filler is just a powder you mix in to the wet epoxy to make it thicker. It is a little like adding flour to a wet dough mix when baking. I was told that it should be of "peanut butter" consistency for most filling applications (holes, etc).Here is a description of the product from the West website:Fillers are used to thicken the basic resin/hardener mixture for specific applications. Each filler possesses a unique set of physical characteristics, but they can be generally categorized as either Adhesive (high-density) or Fairing (low-density).Adhesive filler mixtures cure to a strong, hard-to-sand plastic useful in structural applications like bonding, filleting and hardware bonding.Fairing filler mixtures cure to light, easily sandable material that is generally used for cosmetic or surface applications like shaping, filling or fairing.