Also, use a different color epoxy for your first primer coat than the color you use for your build coat. For example, if you're using a grey build coat primer, use black epoxy. That way, combined with your guide coat, you'll know when you've gone through the build coat to the epoxy, meaning that's a high spot. You can use anything for the guide coat. If you have some red primer lying around for example, use that on your grey build coat, just anything that will give contrast and show where it's sanded off and where it's not.