Dark Wood Side Table How To Re-stain Already Stainded Wood?

How to re-stain already stainded wood? - dark wood side table

I bought a coffee table and side tables. The wood was stained and coated with a glossy sealant. Now it is much clearer than I would if I think back to the office so that they become darker. I wonder whether you remove the original stain and varnish? Or can I just sand and then do spot?