decoupage question

[dopey004.livejournal.com]

hello all, i am about to start my first project using mod podge. i want to do an altoid tin for my boyfriend of his fav. bands and stuff. ive started cutting out pics from magazines. my questions are:
do you have to spray/paint the tin with a primer or paint first? or do you just start with the tin the way it is?
once the tin is finished, say he finds another pic to add to it, can you just add that the the tin or once its finished its finished?

im sure ill have more questions later on when i get started. thank you all for your help and inspiring me to become crafty ;]

-dopey