Using Flex Seal inside and out?

[Chris2018]

I think before spending money on Flex Seal I would go with Pond Armor Pond Shield.
Pond Armor is a Non-Toxic two part epoxy that can be diluted with Denatured Alcohol and applied in several coats.
Its a little price at 60 dollars for 1.5 qts BUT it is far better than Flex Seal plus you can get it in a number of colors.

Note -
Although Pond Armor will stick to and seal plywood water tight (people use this stuff to make plywood aquariums) it is not recommended on OSB.

 

[Appalachickens]

Would polyurethane work to help protect/waterproof a coop?

 

[Chris2018]

Would polyurethane work to help protect/waterproof a coop?

Polyurethane doesn't truly waterproof wood it will make the wood water resistant BUT you would have to re-coat the wood often and wont hold up well were the chicken will be walking and scratching a lot.