Painting ? Very much a personal decision.
MB doesn't paint. In contrast, I choose to paint both inside and outside.
The reason I paint the inside of boxes is because a beehive interior is a very wet environment indeed - just like a shower room - with condensed water running down the walls. Internal painted surfaces assist with this condensation, and also help prevent wood from absorbing moisture, and thus losing some of it's insulation value. It also helps to stop the woodwork from swelling.
A popular view is that there's no need to paint the inside, as the bees will propolise the interior themselves - which is true ... eventually. But they won't propolise the interior during their first year of occupation - they have far more pressing matters to attend to. So I paint, and they propolise over that paint.
With regard to the paint itself - I use whatever's available. Polyurethane floor paint is my favourite, as it dries quickly and is as tough as old boots.
If you do decide to paint, just make sure the paint you use has 'seasoned' well - a week or two outdoors - and thus has little or no solvent off-gassing before bees are installed. If in doubt, leave newly painted boxes outside, and when scout bees can be seen investigating them, that's a sure sign that bees are finding them acceptable.
LJ