The ClearShield kit finished first overall, and the reason is hardware rather
than chemistry. Its bridge seals against the glass with two O-rings that act as
piston rings, so it can both push resin in and pull air out — it was the only
assembly here that could lift a curing strip by vacuum alone, and it was the
only one that drove resin the full length of a long crack including the legs.
It also comes with a drill and enough consumables for many repairs. At $319
that is a tool for someone repairing windshields regularly, not for fixing one
rock chip.
Among the DIY kits the Permatex is the pick at $13: a clean, well-written
procedure, a resin that outlasted the glass at 20.6 lb, a number 3 pick needed
to scratch it, and an adhesive pad that released far more cleanly than the JB
Weld pad. The JB Weld kit costs the same, has the clearest instructions and the
largest syringe, and produced the strongest resin of the whole group at 23.6
lb, but its repair left a small amount of visible damage after 35 minutes and
the resin's tendency to migrate outside the repair was the professional
technician's main criticism of it. For sheer value the Rain-X kit is hard to
beat — about 10 repairs for $12, resin in the top group on both bench tests,
and the sturdiest of the three suction-cup applicators.
The kits to approach with lower expectations are the UNIIOON and the Athlon
Tools. Both use the same style of suction-cup assembly, and on both the
pressure bar behaves like a funnel instead of a piston, so the resin has to be
coaxed into the break over repeated attempts — the UNIIOON took about twice as
long as the Rain-X and left the damage quite noticeable, and the Athlon Tools
never reached the legs of the crack at all. Their cured resins are also the two
softest and weakest here, at 8.6 lb and 4.6 lb. If the damage is a simple
bull's-eye and the goal is to stop it spreading, a $13 kit with a working
injector will do the job; if it is a long crack, the gap between a DIY kit and
professional equipment is real and visible.