In general, epoxy resin systems can be sanded, once cured. However, cured epoxies leave an amine blush that appears on the surface as a waxy substance. This substance can not be removed by sanding, but it is water soluble and can be washed off easily.
Before sanding cured epoxy, use water and an abrasive pad to remove any amine blush, rinse, and allow the surface to dry completely. If you used peel ply to prevent amine blush, you must remove the peel ply before sanding.
The sandability of mixed epoxy depends on whether you used plain epoxy or added fillers to the epoxy. For example, mixed epoxy thickened with milled glass fiber, colloidal silica, or a combination of the two is not sandable after curing. However, colloidal silica mixed with microballoons has some sandability after curing. Epoxy filled with microballoons is the easiest mixture to sand, once cured (with phenolic microballoons easier to sand than glass microballoons).