There are many factors that can contribute to mold rubber not curing including cold temperatures, contaminants on a model’s surface, etc.
But the most common reason: inadequate mixing of Parts A & B. People mixing rubber for the first time tend to under-mix the material before applying. It is especially
easy to under-mix thick materials like silicone or brush-on rubber. Mixing large volumes of material also poses the risk of inadequate mixing.
Mixing Success: A Function Of Time & Technique – Time – Most mold rubbers give you plenty of time to mix and apply, so don’t worry about not having enough time.
Your goal in mixing is to be thorough, not fast.
Technique: Don’t Be Casual, Be Aggressive – This is where most “first timers” make the fatal mistake of using a “limp wrist”, meaning that they are not aggressive enough
in mixing the rubber. Use a stiff mixing paddle and employ a firm wrist, making sure that you scrape the sides and bottom of your mixing container several times.
Important material hides on the bottom and sides of your mixing container and failure to get this material into the mix will result in the rubber not properly curing.
The Best You Can Do -
“The Double Mix-And-Pour” Technique - After mixing thoroughly for 3 minutes, empty the contents of your mixing container into a second clean mixing container and
mix again for another 2 to 3 minutes before applying over your model. This technique is your best bet for successful mixing and the protection of your mold rubber