Varnish for Ceramic Tiles?

[whitestripe]

I have recently tiled an old coffee table with ceramic tiles.
As the grout is white it will probably at some point get stained, so I thought I could possibly varnish the tiles and seal in the grout.
Does anyone know the best type of varnish to use. It needs to be durable, without the risk of cracking or crazing.
Cheers.