I have never done this because I don't have the equipment,
http://www.bushfarms.com/beesdipping.htmwhat I have done is painted with "boiled linseed oil" heated enough to melt in some beeswax, painted on hot. Latex paint from the mistint section is easier and cheaper, for me.
On the inside, I have I have an oscillating tool, and a rasp bit for it, and I scour the inside of the box to encourage the bees to deposit propolis. Cf. T Seeley's recent research. While I'm at it, I route a 1/4" groove in the center of each short side, for the purpose of just-in-case I need to put in a divider board someday, I don't have to route inside an assembled box, and I don't have to go searching the yard for a box with a groove, because they all have it. :)