Stubborn Crack Issue Finally Resolved?
After looking a little closer at the area where the unfix-able crack keeps appearing, I believe I finally see what the problem is. The helmet plastic is slightly flexible, while the Bondo used to fill the dent is more ridged. If there is any flex, at all, in the dome plastic, the Bondo doesn’t flex with it, exposing the crack between the two materials. I am going to attempt to get rid of that material incompatibility by super gluing them together at the seam.
I super flexed the dome, to cause the parts to separate. Then, I put liquid super glue all along the cracks, letting it seep in via capillary action.
Once it was good and dry, I sanded off the excess glue.
Then, added another (hopefully, the last) layer of glazing putty.
Sanding, after the glazing putty was dry.