Doug,
I think that modern car clearcoats are polyacrylic urethane. Chemical resistance is about the same as with polyurethane (at least according to my experience with polyurethane paints): no problem at all with methanol, nitro or acetone.
Again according to my experience, polyurethane is slightly easier to sand & polish, but propably there are differences between different brands.
For example, untill now I have used (Acrylic clear, that is) the Italian brand Sestriere. After spraying I sand everything with #2000 and then polish by hand with Farécla G10. It’s hard work because it’s impossible to respect the 24h after spray -polishing that is recommended. But it’s doable anyway, we have a long winter.
Now, I tried a PPG clear (D800) and it’s clearly more difficult to sand and polish than the Sestriere, even though they share a similar chemistry. Maybe it's because of the very high solid content of PPG? I think I’ll have to go with PTG style and sand with a little rougher paper and then spray one more highly diluted coat of clear and continue finer sanding & polishing from there.
I really like the polyurethane (cheap industrial 2-component) for base colours. I prefer them from 2-component car paints as they have no plasticizers, meaning they are really nice (hard) to sand and polish too, if necessary. L