First the disclaimers:
1) I don't work with or sell Zero Rust
2) I'm not associated with Autobodystore.com - I just hang out there
3) I've never conducted a double blind study
4) I don't own a Volvo
5) All opinions are my own

Isocyantates are a real concern. One time exposure can result in lifetime changes to your life and there is no way to predict who will be affected. Here is the government information on isocyanates.

http://www.cdc.gov/niosh/asthma.html

By the way, remember a few years ago when hundreds of people died in their sleep in India. That was a release of an isocyanate compound.

I have used Por-15 (P15) and Zero Rust (ZR) in auto restoration work. They both fill a need and they both work as advertised. Since I find ZR a lot easier to work with, I buy a lot more ZR.

I have experienced lifting problems (peeling paint) with P15. ZR hasn't caused me that kind of problem.

Unused P15 will dry rock hard in the can. ZR doesn't.

I have found ZR offers better support to my questions. That's probably not a real issue, after you have used it a few times, but it certainly was in the beginning.

P15 has isocyanates. ZR doesn't. Again not an issue if you have a fresh air respirator.

ZR comes in a rattle can. P15 doesn't.

Now for some personal testimony.
I painted some steel fence posts in 1997 with 2 coats of ZR from a rattle can. They have been exposed to the Pacific Northwest weather since then. Only prep was a wire brushing and degreasing. Not a speck of rust has appeared. Not bad when you consider it rains for days on end here.

Not really a recommended thing to do but I painted an entire pickup with ZR. The prep was a sanding with a DA sander and degreasing. The owner was planning to paint it in the future but he was so happy with the results he just left it as is.

I use ZR for undercarriages, fenderwells, inside of door panels, etc. It stops the rust and leaves what I consider just the right amount of semi-gloss for these areas. I thin the ZR with lacquer thinner and spray it with a HVLP gun. It doesn't get much easier than that.

If you have questions about autobody work and painting, you might stop by the autobodystore.com message forum. You have people there that have decades of experience and are more than willing to give away their deep, dark secrets of the trade.