I haven't had any issues with shrinking or cracking. I like to use it mostly to reinforce joints similar to a welder would use weld. I use it to reinforce joints when I make a roll cage or I will brush it on the backside of a joint that has very little contact surface. I also like to use it to build up edges, for instance if I need to change the shape of a wheel opening. I let it dry overnight then I can sand it just like virgin plastic.
I built up the edge of this air dam (the white plastic) then sanded it to shape after it setup.