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Painting Your Plastic Casting
You’ve made the perfect mold and have 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 the paint 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. Not using a release agent is not an option when your mold is made of urethane rubber, and using a silicone rubber without release agent limits the production life of the mold by 1/2 if you are casting urethane resin.

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 ATH or talc following the application of a release agent is one way.

Click HERE to learn more about the powder coating technique . . .

The Best Method Incorporates The Use Of A Spray Primer Applied Directly To The Mold Cavity Prior To Casting.

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.

Plasti-KoteTM Sandable Primer - can be found at auto supply stores in the U.S. Contact: Plasti-Kote in Medina, OH Tel. 800 431 5928 or http://www.plastikote.com/products/Primer/Sandable-Primer.html

BulldogTM Adhesion Promoter - Contact: W.M. Barr Co. in Memphis, TN Tel. 800 398 3892 or www.wmbarr.com. Or order online at www.levineautoparts.com

 

Here’s How . . .

primer1_141 primer2_188 primer3_167
1) Mold release is sprayed into the mold cavity. 2) Auto body primer is then sprayed into mold cavity. 3) Resin is cast or sprayed behind the primer.
     
ready_to_paint_200 primer5_206
4) Casting comes out of the mold
ready to paint.
5) Spray paint then adheres to
casting surface without a problem.
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