I dullcoat over them.... 2 reasons:
Dullcoat gives the model a bit of "tooth" so you can do weathering with chalks. Also, it makes the finish even.
After weathering, the whole thing gets hit again with dullcoat to fix the chalks.
I don’t sand them, I just white glue them on (white glue softens the paper so you can press it into the building details), but it depends on the paper, I suppose....