I've had much better experience. I used stop leak successfully on my somewhat older Ford vehicles to seal minor leaks. It stopped the leak, and I never needed to add freon again. Had no hardening or other issues. In fact, I gave my 91 Taurus to the nephew 5+ yrs ago and his family is still driving it....and the AC works.
Just want to add, the 91 used R12. I added R138 stop-leak to my 98 Windstar with the same positive results.