After learning more about these (unexpectedly, too) from below, I started looking more closely at every crosslinker or second part (catalyst?) of two part finishes.
I see there are conversion varnishes that are waterbased (always guess that the name could be misleading when it's WB) and one of the ones mentioned was suggested as having very very good hardness and scratch resistance.Upon further reading, the catalyst said "for professional use only". tracking down the SDS shows some type of isocyanate, can't remember the exact name.
It appears that just about every very hard finish that I'd want to use to put an automotive style finish on a guitar (very hard finishes that are also scratch resistance with good toughness), other than just putting lacquer over paint, seems to have isocyanate in the catalyst - including the two part waterbase finishes.
That led me to wonder what's in crosslinkers, as I generally use the crosslinker with target WB finishes because it's soft without it. Target recommends finish only without crosslinker on guitars, but i don't agree with their recommendation as I'd prefer harder and less thick. Neither here nor there except making sure that it's something OK to spray in the garage. (it doesn't appear to have anything similar to the items mentioned above, rather "Organosilane Ester", which doesn't turn up anything significant for carcinogenicity).