I can't tell from your profile where you are based, but I love the Tung Oil finish on my red cedar hives - you just apply a couple of coats to the outside, using a wipe or paint on coat, wait 30 mins, then wipe off the excess. Allow to absorb for a couple of days, then apply another coat in the same way. For the first coat, I diluted 1:1 with food grade D-Limonene, because the oil absorbs slightly more evenly and quickly, but this is not essential. Enough Tung Oil and D-Limonene in the US is about $12 per hive. You only paint the outside surfaces - parts that bees don't walk on (but I do the landing board too). It leaves you with a very natural, slightly darkened finish. It should last a couple of years before needing another coat, and all of the products are totally non-toxic.
If I could, I would dip the hives in wax/rosin, but I don't have the equipment and I don't have enough hives to make it worth investing in it. That gives you a 10 year+ finish.