What is this paint? and what should I do about it?

[Campbel45]

Guys,

Attached are a few pictures of my 1966 (maybe 64) GMC L4000 tilt cab. Someone has obviously painted over the the factory white. The green surface paint is very dry and cracked. What kind of paint is this? Why does it crack like this? I doubt it can be saved, so could I sand down the top layer and go with a primer+paint?

In some spots, there is some light rust getting through. I know I will need to sand down to get rid of the rust spots in those areas.

Sorry if these are beginner questions and observations...but I am one when it comes to paint!