NOTE: The above article only mentions one Resin "Ecklers R-916".
Ecklers no longer sells any under this number. Also this may cause
some confusion as there are two different types of glass from '68 to '82.
The '68 - '72 cars have GRP glass like a boat which is rough on the back side.
The '73 - '82 cars have SMC which has a marble like appearance and is slick
on both sides of the panel.
You CAN NOT use the same "Resins" for both types of glass.
Even though in this article it leads you to believe you can.
Be sure you buy a Resin that is compatible with the type of glass you are working with.
Neither type had gel-coat installed from the factory.