Wait... you compare isotopes to car paint jobs? So students mentally categorize isotopes as things that are functionally the same but look a little different?

It's an analogy that will only take you so far. Consider Carbon-14, for example. If your student hears that Carbon-14 is radioactive, will they assume that all other Carbons are radioactive, too? Adding a neutron to an element does more to it than adding a new paint job.