You have made the perfect mould and just poured Smooth-Cast™ liquid plastic into the mould to make a reproduction. The plastic cures, and you demould 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 mould, for example, makes painting a casting almost impossible. Removing release agent from the casting surface can be difficult and is another labour step.

If you are using a urethane rubber mould and casting urethane resin, you must use a release agent to facilitate demould. Using a silicone rubber mould without release agent limits the production life of the mould by half if you are casting urethane resin.

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 demould.

Powder Coating Technique

Powder coating the mould with URE-FIL™ 7, baby powder or talc following the application of a release agent. Here is how it works:

Prior to casting the resin, Ease Release™ 200 is applied to the mould followed by a powder coating of Ure-Fil 7. An air hose is used to blow any excess powder from the mould. The urethane resin is then cast into the mould. The casting comes out of the mould with a dry, matte finish. An auto body primer is applied, followed by an acrylic paint and finally an acrylic sealer. The major advantage to powder coating is that there is no release agent to remove when the casting comes out of the mould. Models can be immediately primed and painted.

Apply Primer To The mould Surface

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 mould 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 mould ready for painting.

Which Primer Should I Use?

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 mould surface or applied to cured castings (no release agent used), two primers did stand out, providing excellent adhesion and the best for painting.

• Plasti-Kote™ Sandable Primer
• Bulldog™ Adhesion Promoter

What type of paint should I use?

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.