suitable varnish?

Hello - I'm new to the site and new to chickens!

We've just bough a hen house ready for our first three hens - hopefully arriving after Easter. I read somewhere - but can't remember where - that we should varnish the inside of the house (built of pressure-treated timber) to make it more difficult for mites to take up residence and then let is air for a couple of weeks before the hens arrive. Is this necessary/a good idea? If so, can we use ordinary varnish or might this be bad for the hens? What else could we use?

Any help much appreciated! Thanks.....

Owli