[HO455]

I believe the problem with cracking comes from the hot and cold expansion/contraction cycles. GM's original formula seemed to match the expansion rate of the steel/iron frame of the wheel but over time the plastic hardened up as the solvents worked out. In order to get a lasting repair you would need to match the expansion and contraction of the wheel material and that of the product you want to use.
I would use a test wheel and figure a way to be able to accurately (and repeatedly)measure the changes with temperature. The longer the test section the easier it will be to see the changes with your micrometer.
Then make some test pieces out of the product(s) you plan to use that are the same length (and mimic the general shape) as your test sections on the wheel. Place your wheel and test pieces in the freezer for a half hour or so and measure them. Follow this up with the heat test at least 150 degrees in the oven and measure. The closest match in explansion and contraction amount should be the most resistant to cracking.
Just my opinion your results may vary.