When clear, liquid epoxy products are sanded, they lose their clarity and take on a whitish appearance. This is normal. Clarity will return once additional epoxy has been applied over the sanded surface. Similarly, in some applications, the epoxy is sanded in preparation for a UV protective topcoat. After the topcoat product is applied, the epoxy will once again take on depth and clarity.
Another approach, though very labor-intensive is to continue sanding up the grit chain to the point where a high gloss appearance returns. As well as being labor-intensive, it can also be quite expensive, easily running into the $100's of dollars. Depending on where you start sanding, you will likely invest in upwards of 8-10 different grits not to mention multiply rubbing compounds. Meticulous detail is required for a successful outcome.