Hi Frank are you looking to varnish the mantel or the entire surround, is it is the surround what is the material?
Is it brick or tiled?
What on the fireplace needs to be varnished?
You cannot varnish bricks- the finish will not withstand heat. Scrub with TSP to clean them up.
You can only varnish the wood, perhaps a mantle is wood.
I agree with Lisa. You need to say what you want to varnish.