i have a question that is pretty much identical to the forum question about "erasing" pixels (which proposes lerpColor(), but doesn't work for me)

my inspiration is the erosion of paint layers on the outside of buildings. mainly the peeled effect that shows the different built up layers that have been applied over time.
i would love the user to be able to "erode" at the top layers of a stack of images to eventually get to the bottom one using the mouse. ideally i would like the rings of the different layers to peek through.
(would also love to add the passing of time to build rust over newly worn away spots...but i already feel like i bit off more than i can chew right now)

i am having a hard time achieving the above mentioned. i have tried many options:
copy()
mask()
blend()
alpha()


here is what i have now http://itp.nyu.edu/~lg1285/ICM/midterm/index.html

can't figure out how to get ring effect or go through more layers though. i am thoroughly stuck.

would like to try a different method.... any suggestions?

thank you for reading