Tiny bubbles/pinholes when painting resin

[superrune]

Hi everyone,

Sorry to post a non-replica item in the forums, but my Star Wars models tend to be all virtual these days. But I've got this problem on a batch of resin models that I have been painting the last week, and it seems to me that you guys would be the best help on this.

I've printed out a couple of 3D models at Shapeways that I have molded in silicone. After that I cast out a bunch of resin copies using Smooth-On 305. So far so good, everything looks excellent. My tiny problem appeared as I tried to paint the resin models.

When spraying the resin, either with acrylic car-paint, primer or synthetic paint (montana spray cans), I get these really tiny pinholes along corners of the resin. They are quite small, but they stay after each coat, getting deeper and darker after each layer is painted on.

Here's a photo where you can see them:

I haven't used any kind of release agent on these models. And while the models are smooth to the touch, they are not oily or wet in any way. Unfortunately, I'm spraying outside, in quite cold weather. But the paint is kept indoors, and the models are drying indoors as well. Anyway, I'm also painting some metal objects at the same time, and they don't have the problem, which makes me think it might be the resin. The primer sticks really good to the resin though, so it's not as if the surface is impossible to paint or anything.

Has anyone here had this problem when painting plastic or resin? Any tips to what I should try to do?

Thanks!

 

[GKvfx]

It appears that the pin holes are actually in the resin castings themselves. Are you using a pressure caster? For those types of bubbles, I usually brush on some Mr. Surfacer and then lightly sand the area in order to get it feathered in with the rest of the casting. Then I paint.

Gene

 

[Bigturc]

I've had the exact same thing happening to me on one of my Zorg resin droid head .. and only one. Funny thing is that only 1 of the 3 did that .. all coming from the same batch, all painted at the same time, with the same paint. Note that all of them had been properly washed with soap before primering too.

I was suggested to sand down the defect area and reprime over it. I have no idea what did that .. it was certainly not holes in the casting .. as it was flawless and really smooth (thanks Chris).

I have never experienced that again on anything else!

 

[superrune]

Thanks for your feedback.

KBvfx: They could be in the castings themselves, but since the resin is so translucent, it's very hard to see. When I spray, I do it with multiple layers of primer. Shouldn't that cover up any small bubbles on the model? I'll try your trick of brushing and sanding the models. Any tips on how to sand these super-small details?

Bigturc: Sounds a lot like what's happening to me. In my case it's more two out of three that comes out with tiny holes. I was hoping on saving time by skipping primer and sanding, by spraying car paint directly on them (the car paint seems to stick really well). But with these bubbles I might have to prime and sand all 100 of these devils (with seven times as many ready to be cast)...

Oh yes! One more thing - new bubbles/pinholes appear over the primer as well, even if the primer is smooth! Anybody seen that before?

 

[modelcitizen]

all too common in resin. this is why i don't do too many more resin models
sometimes if i sand and don't clean off the dust too well before another coat of primer they fill...sometimes not

 

[actias]

The pinholes are caused by moisture in the resin and/or on the surface of the mold when the humidity is high. One trick is to coat the mold with a thin layer of baby powder (The cheap kind - must have corn starch in the ingredients). Just shake the powder in and bounce it around in the mold. Then dump out the excess. Also keep the resin sealed as soon as you pour what you need. If you leave the caps off the resin containers -even for a short time- the resin literally sucks the moisture out of the air.

 

[superrune]

Actias - thanks!! I will try that out! How about spraying primer into the mold before casting. Could that be another solution?

And is there anything I can do with the resin I have already cast?

 

[fettster]

Actias - thanks!! I will try that out! How about spraying primer into the mold before casting. Could that be another solution?

And is there anything I can do with the resin I have already cast?

another thing it could be it the the silicone mold, if its areally fresh mold,sometimes the silicone can transfer some of its oil to the casting, which is hard to remove.

try washing the piece off with naptha befroe painting but the primer in the mold should also work

 

[actias]

I have done the primer thing and dont particularly care for the results. You'll find that using the powder (Without mold release) will give you an almost velvet like finish. You will still need to wash the part, but the paint will stick like glue. I have spent many hours molding and casting (did some work for Trek The Experience in Vegas and prop replicas for Starland in the early 90's) and have found the powder method to work the best. In fact the Japanese resin manufacturers use this technique which is why their castings are so nice. If your resin has moisture in it already, there is a product you can buy a (Called Desicant - I think) that removes the moisture when mixed in with the resin. You'll have to do some online research or call the resin manufacturer for more info, but it does exist. I never needed it because I kept everything dry as possible.

As far as the stuff you already made, you can buy a product that is like spackle and you just rub it in over the pinholes. I have a tub of it here but have to dig it out to get the name of it. Give me a day or two and I will get the info to you. Hope this helps.

 

[superrune]

This is great help, thanks a lot Actias! I will definately go with powder on my next batch. I've ordered 7kg of resin, and I was anxious about this happening to all our next castings (which will be around 1000 pieces). If the powder works, that will really save this project.

Fettster, I think I have some industry oil remover lying around, I will try that too.

 

[RickDTM]

If the pinholes are in the casting itself, the easiest way I've found to correct this is to spread some Bondo glazing putty over the area, then smooth it out w/ some acetone on a Q-tip. This melts the putty into the holes, and usually a light sanding is all that is needed to finish the piece when the putty dries.