I am going to papier mache over banker's boxes for attractive storage in our living room (so I can just quickly throw things into when company comes!  )

I have made papier mache storage boxes before out of just regular leftover cardboard boxes with wallpaper paste and pvc glue that I painted with arcrylic paint a few years ago - but they are worst for wear now, and I can't clean them without the paint coming off the boxes since I only sealed them with white glue.

What is the best thing to seal paper mache with so you can get the dirt of of it  as time passes?  My older papier mache boxes are also showing tiny cracks in places. Is there any way to prevent this in the future as well?