WHY does this stuff always crack?

The fiberglass seems better.

Using too much hardener or putting it on too thick

Cool. I was having this discussion with a friend who is also into restoration. For me light application, but he goes nuts with the stuff.

Used correctly it won't crack. It gets a bad rap from amateurs who don't know what they're doing and then blame the product when the repair fails.

Yeah its supposed to be used to smooth bodywork not BE bodywork lol. Also should never use to fill holes

Absolute max thickness iirc is 1/16" of an inch.

you're not supposed to sculpt a car out of it. i've seen some really creative sculpture... saw a car recently that had at least 10 pounds of bondo on it.