Eco-Safe Wood Treatment is extremely popular for apiary use because it doesn’t negatively affect bees. Most of the time our product is applied to empty hives, so the entire structure can be treated, inside and out - we often see customers dipping all sides of the boxes right into the solution to achieve full coverage, letting the product dry, and then rinsing well.
If your hives are occupied, you can still treat them, but of course you wouldn’t be able to get to the insides.
If you’re treating hives in use, brush the product on the outside, rather than spraying, so you can carefully control the application. The liquid is very thin, so you won’t need much on the brush at a time - you could also use a rag for application.