I think it is worth saying to the "readers" that we would only coat the OUTSIDE of the box, to avoid having chicks in contact with potentially toxic substances. This is the standard practice for bat boxes and bird boxes alike.
I should think that "pet-friendly" products are best - that is, water-based ones, designed for garden exterior DIY. I think that "Cuprinol" boast this attribute for certain coating products.
I see that Sadolin has already been suggested. Again, a multi-coat approach is probably best. Lots of people use this on timber outdoors, but depending on original hard work / number of coats, you periodically have to re-coat.
I am also open to suggestions!
PS: I wonder what our box suppliers use? (CARECROSS)