Bookcase Finishing questions

Hello, I have a few questions about finishing a bunch of bookcases I am working on. I have made these bookcases with shelves on the front and back. (They are going in my bookstore which is opening next month.) They are 60" tall, 48" wide and 25" deep. (About 12" deep on the front and back.) I am planning on using a clear or mildly tinted stain on inside sides and shelves of these bookcases and painting the back and outside sides black for a contrast. (I am using 3/4" "Radius Pine", a very nice plywood on one side and adequate on the back. The Good side would be used on the inside of the sides, because very few of the outside sides would be visible as the bookcases would be lined up in a row, and I will be finishing the end bookcases a little differently.) I was wanting to use some sort of clear varathane or other protector on top of the stain to protect it. My questions are: - What is a good protector, or should I just use varathane? - My wife saw some exterior-grade deck varathane(?) on sale for a great price by the gallon. What kind of difference is there between the exterior and interior? (My inclination is to stick with interior, hers it to say that if it is exterior it will give more protection.) - Is it a good idea to use the varathane on top of the paint as well to give it more protection? (I don't want the paint to wear off onto the books.) - Please keep in mind that these are bookcases that are going to see a lot of use, and so have to stand up to more handling than most bookcases. (Because they are going to be in a store.) - Any other finishing suggestions would be greatly appreciated. (ie: how many coats of varathane I should use.) :) Thanks, Sean.

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