TIP: Painting Smooth-Cast 300 castings

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New Member
Now that Hez has posted an excellent tutorial on bio molding and casting, I thought I'd post a hint for anyone who's considering working with Smooth-Cast 300 or some other urethane plastic material. (In other words, I'm telling you what I learned the hard way so you don't have to repeat it.)

Painting a piece cast in Smooth-Cast 300 can be rather difficult. I'm not sure if the smooth surface makes it hard for paint to stick, but if you use a release agent on the mold, that makes it almost impossible to paint. I found this FAQ on Smooth-On's site that explains the process. Basically, you spray the inside of your silicone mold with a release agent, then brush baby powder or talc over the surface. You'll need to vacuum up the extra baby powder. After casting the plastic, you end up with a nice matte finish that will accept paint.

They also say in their FAQ that there are only a couple good primers that will stick to the plastic. I found Plasti-Kote Sandable Primer at my local CarQuest store. (I couldn't find it at Auto Zone, despite what the FAQ claims.) That's been working well for me. I previously tried a lower-quality primer, Dupli-Color Adhesion Promoter, but that didn't work at all. The paint beaded up on the surface when I tried to spray it on. I don't think Dupli-Color Adhesion Promoter promotes anything adhering to anything.

Anyway, that's my helpful hint. (Well, that's Smooth-On's helpful hint, but I've tried it and it works for me.)

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New Member
Thats some good tips. I take the pieces and wipe them down with MEK. Its a solvent that removes pretty much any kind or type of residue. What also helps is to just scuff the surface with a scothbrite pad then wipe down with MEK. I havnt had any problems with paint adhesion.


Well-Known Member
I myself use smooth-on 300 series and my molds are most of the time made from rebound 25 which is a platinum cure silicone. I dont use a release agent and have never had any problems using a platinum silicone. I stay away from tin cure so cant say anything about the stuff. As far as painting the plastic i use plasti-kote paints most of the time.


New Member
i use this same resin and if you wash your parts in soapy water they paint just fine.i love the duplicolor adhesion promoter as well.plus their sandable primer is the best primer ive ever used,ever!

that release agent leaves a slippery film on it.i just take all my parts to the sink and have a little hot water soap party and they clean up real good.if you have some fine sandpaper you can even get em a little work over in the hot water.

now that adhesion promotor HAS instructions on the can,you gotta spray it on and wait the 10+ minutes.then you can begin adding your paint and primer.ive painted all my plastic armor with this stuff and it works awesome.if you spray it on and then immediately paint then it doesnt work.ive got all my pred armor ,my ironman armor,Halo armor,2 sets of clone trooper armor and more(yes i have a bunch).i wont paint plastic without that adhesion promoter now! so try it for yourself!


New Member
I know... bumping an older thread... but it's a tip thread.

I will use mold release only when making molds where silicone on silicone is expected and even then, sparingly. I don't use mold release on my silicone molds when molding for the same reasons as skipjenn. Use too much and it effects the surface details too. Now with urethane rubber molds... use mold release... every time!

On my larger molds I baby powder the surfaces complete with a brush and use compressed air to blow out the extra. Baby powder reduces the surface tension and therefore reduces surface bubbles. That's really important on the long thin molds for say... blades. It's supposed to help lengthen the life of the mold too.

I've read that baby powder helps the surface for paint adhiesion but I haven't seen a big difference. Green Scotchbrite has worked well for me on surface prep too.

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