You have made the perfect mold and just poured Smooth-Cast™ liquid plastic into the mold to make a reproduction. The plastic cures, and you demold the piece to reveal a casting that looks exactly like the original. Your next step is to paint the casting for outdoor display, but when you try to apply the paint it beads up on the casting surface and won't stick.
Urethane plastics can be hard to paint, and there are a number of variables that can affect the outcome. Using a release agent to release the casting from the mold, for example, makes painting a casting almost impossible. Removing release agent from the casting surface can be difficult and is another labor step.
If you are using a urethane rubber mold and casting urethane resin, you must use a release agent to facilitate demold. Using a silicone rubber mold without release agent limits the production life of the mold by half if you are casting urethane resin (related: FAQ: How can I remove release agent from my casting?)
Pro Tip: There are a couple of ways to get the benefits of using a release agent and still have a casting ready for painting following demold.
Powder coating the mold with URE-FIL™ 7, baby powder or talc following the application of a release agent. Here is how it works:
The other way is to use a spray primer. Here's how it works:
Following the application of a release agent, an auto body primer is applied to the mold cavity. Casting resin is then mixed and poured or sprayed. The auto body primer bonds to the casting's surface and comes out of the mold ready for painting.
Customers contact us regularly asking which primer works best as a foundation for painting their Smooth-On plastic castings. To settle the question once and for all, we tested over 30 aerosol primers and found that most did not work, or worked poorly at best.
The low-quality primer would "bead up" on the surface of the casting.
Whether applied to the rubber mold surface or applied to cured castings (no release agent used), two primers did stand out, providing excellent adhesion and the best for painting.
After primer has completely dried, the casting may be painted. Acrylic paints generally work best. Model train paints (available at many hobby stores) work well. Some testing for paint compatibility may be necessary.