What should we apply to our new coop?

I know next to nothing about chickens but from what I've read on here - fill in all the nooks and crannies with sealant. What, I don't know, presumably something which hens won't eat.
I'm sure a hen person will be along soon to advise.

I've not used any sort of sealant and (touch wood) haven't seen any mites yet. If you pick out any poo every day, clean out the straw and shavings regularly and check for any unwanted visitors eevry time you look in, you should be ok. If they do show up, you'll need something on hand that will effectively deal with them.
I have poultry shield but there are stronger treatments available. A lot of keepers use Diatom in the coop as a preventative.

You can read all about it here...

http://chat.allotment-garden.org/index.php?topic=5250.0

Rob