If I have sap left that I did not get to boil I fill the pans to the top with the sap. I let it sit for a month or so and then dump it out and they are shiny new!
The sap basically turns to vinegar after sitting so long.

It's not looking like I'll have any extra sap this year so I'll put a couple of gallons on white vinegar in the pans and then fill to the top with water an let sit for a month or so.