Great question! Clear epoxy can be applied on top of already finished wood with a stain and clear varnish on it if it is completely dry. Sand with 220-grit and then proceed with your seal coats and flood coats. Here is a video showing how we did it. You can apply over stain without varnish if the stain is water-based.
It is always a good idea to do a test board before you pour. We do highly recommend using just the epoxy on bare wood as it’s a beautiful look on its own and oftentimes folks choose to save the extra time and money on a stain!
Thank you for your great question and contact us anytime for free project support. Have a great day!