Dark patches at the edges of cork tiles

[Avocet]

I wonder if anyone can help?

I have put down some cork tiles (Wickes own pre-sealed) on the landing. After laying them, some of the edges were very slightly proud of the neighbouring tiles in a few places, so I gave them a "lick" with some sandpaper. I have now put a couple of coats of varnish over the whole lot and have noticed that the areas that I had sanded have come out darker than the rest! Does anyone know what I've done wrong and how I can fix it?

 

[wingcoax]

The sanding has removed the original sealing and the varnish has penetrated the cork, unfortunately there is nothing to remove this.