Every shop should have shellac on hand. It’s the aspirin of wood finishes.
General info on basic clear finishes.Zinsser shellac is readymade and readily available. (You can make your own with dried flakes and alcohol from heavy to extremely thin for different purposes). But for sealing wood the ends of the boards it would take a few coats to equal one coat of varnish.
For the board ends I’d just get a quart of any varnish and a disposable bristle brush.
For “cleaning” the boards I like the RAS115 suggestion. It would be good practice to learn how to be productive
and maintain fairly good sustainable collection.