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Can this be used on top of current stain? Also, do you have a medium or dark brown color in it?
As long as it is a stain that is down and not a lacquer or varnish then yes it will be fine.
You can find the colour chart on the product page linked below.
http://www.adsealcolours.co.uk/NewDeck-with-CoolStain-Technology.html
Drew Palin