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Pinholes In My Resin Parts-HELP


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Recently I`ve encountered pin holes in several resin part I acquired. The parts looked absolutely perfect on the surface but, once I began to sand them (with medium grade of wet/dry paper) I began to see tiny pinholes popping up. In fact the more I sanded the more  pinholes popped up. The parts are beginning to look like the pot marked surface of the moon. This is the second time this has happened and the parts are from two different resin casters.

Hence, two questions: 1) What causes this. 2) And, more importantly  how do I fix this. HELP.

 

Edited by 69NovaYenko
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It's caused because the guys doing the casting didn't bother to de-gas the resin during the casting process.

In some cases, the resin itself will release gas if it goes off too quick and "exotherms".

The usual cause is that air gets whipped into the resin during mixing, and if it's not removed or minimized somehow, the parts have the problems you notice.

Some casters place just-cast parts (in their molds) in a "pressure pot", a variably-pressurized sealed chamber which can shrink the bubbles as the pressure rises.

However, the best way to handle the problem is to "vacuum de-gas" by placing the mixed resin in a vacuum vessel prior to pouring it in the mold. Vacuum will literally "suck" the bubbles to the surface, where they pop. When the bubbling stops, the resin is ready to mold.

Lotsa "experts" say it's not necessary to do either, and the result is what you've got.

The reason folks would prefer to skip the step is because it takes longer, pure and simple. A fast-setting resin will sometimes start to gel before the de-gassing process is complete, and it's useless once it gels. Rather than simply using a slower-set resin and de-gassing, some casters use a faster-set resin and just skip the step.

Sometimes you get away with it, sometimes you don't.

There ARE some lower-viscosity casting resins available that claim to be thin enough to release the mixing-induced bubbles to the surface prior to casting, making vacuum de-gassing unnecessary.

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Ace is correct. I do a lot of casting and I always vacuum the resin and use a pressure pot. The resin I use has a 20 to 25 minute

pot life. I'm not sure about the low viscosity resin not creating bubbles, I have a clear resin that's almost like water and it creates more bubbles!

Best bet is de-gas  and use pressure for casting. Also super glue is the way for filling pin holes!

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Be careful using super glue to fix this problem. The issue is the glue is MUCH harder then the resin and it will not sand consistantly. This is bad, especially on a body, as the surface will have waves in it. My best method is to sand out the part then put on a couple of wet primer coats. I use Dupont sandable grey. Let this set up and then sand it again. Re-prime as necessary, trying to keep from breaking through the primer to the resin.

I will not recommend spraying these parts with a "hot" paint, such as laquer. It WILL raise the pinholes through the primer. Use an acrylic or model car type enamel.

Mark

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I don't cast model car parts (OK, I do a few for myself), but I HAVE made a fair number of masters, silicone molds, and urethane copies for product-development R&D. presentations, etc.

One thing I've found helpful when dealing with my own problem pinholes (from when I got in a rush and didn't do it right) has been to screed or squeegee 2-part polyester (bondo) glazing putty into the holes. 

It's sometimes very difficult to get enough primer shot on a part wet enough to fill holes. It can be frustrating to watch them open up again as the primer solvents flash off. You also run the risk of filling surface details by pounding on the primer.

Screeding filler into the holes and then sanding the soft filler off before primering (repeat screed / sand as necessary) has worked very well for me in the past. 

Polyester filler is much softer than superglue, and avoids the "hard spots" problem referenced in the post immediately above this one.

If the holes are very small, you can also screed in a single-part filler like Squadron green, or even a slurry of primer...but because these materials (single-part putty or primer) are AIR-DRY materials, you need to give them plenty of time to harden up, as they don't get much exposure to air down in those little holes.

Another note: Those holes can be significantly bigger BELOW the surface than they appear to be ON the surface. You're sanding a hole in an air bubble, a ROUND air bubble, and a 1/32 inch hole on the surface can have a 1/8 inch bubble beneath it. This is another reason why screeding a filler into the holes may work better than just hammering on the primer, as you can force-fill the bubble.

Edited by Ace-Garageguy
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First many thanks for all the feedback on my pinhole issue. There has been tons of good advise put on the table.

Here is a statics update:

The piece I'm wrestling with is a resin cast hood. Presently, it`s in three coats grey primer. Two thin dusting of Tamiya and a third coat is a medium pass of grey Krylon sandable spot filler primer.  it looks good at first glance but when you get it under  light and closely eyeball the situation you can see several patches of 1/32 inch dimples here and there. These dimples are air bubbles that are right at the surface but haven't opened up yet.

In response to Astroracer advise.."I will not recommend spraying these parts with a "hot" paint, such as lacquer. It WILL raise the pinholes through the primer."

My intent was to replicate an era correct  Nickey tri-power Stage lll car in era correct factory colors so my color coat (which I have already purchased") was  going to be lacquer. 

I am open to all additional guidance at this point.

Edited by 69NovaYenko
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Being a molder too........ I haven't found a lot of my casting to have ANY air bubbles...... it sounds like the sanding process is getting under the top "crust" of the resin, which sanding removes some of the resin opening air bubbles that would be UNDER this "crust"...... IF it were me, I'd try to coat the resin with a hard clear coat first to "seal" it to give a little more meat to the crust......THEN wet sand with high grit wet/dry sand paper, 1200 to 2000 grit.

 

As mentioned tho, pressurizing the mold to suck out air is to make the resin hunker down into the mold itself, removing air bubbles. BigTallDad is right...... when poured, in a thin stream, SLOWLY, reduces the amount of air that can get in the part, through the molding process. Believe it or not, the parts I mold are NOT done with a pressure pot... OPEN AIR pour, with the thinnest stream going in as I can get.....

 

As for the lacquer paint, I have yet to have ANY ISSUE with lacquer paint on my resin parts in fact lacquer is ALL I use, primer or finish coatings.... Not once EVER having an issue getting pin holes.... IF a lacquer clear finish was used to coat it, it ought to be thick enough to fill the holes, not make them! BUT, using acrylic enamels should do the same thing......fill holes not create as acrylic paint is a form of plastic when cure!

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It's caused because the guys doing the casting didn't bother to de-gas the resin during the casting process.

In some cases, the resin itself will release gas if it goes off too quick and "exotherms".

The usual cause is that air gets whipped into the resin during mixing, and if it's not removed or minimized somehow, the parts have the problems you notice.

Some casters place just-cast parts (in their molds) in a "pressure pot", a variably-pressurized sealed chamber which can shrink the bubbles as the pressure rises.

However, the best way to handle the problem is to "vacuum de-gas" by placing the mixed resin in a vacuum vessel prior to pouring it in the mold. Vacuum will literally "suck" the bubbles to the surface, where they pop. When the bubbling stops, the resin is ready to mold.

Lotsa "experts" say it's not necessary to do either, and the result is what you've got.

The reason folks would prefer to skip the step is because it takes longer, pure and simple. A fast-setting resin will sometimes start to gel before the de-gassing process is complete, and it's useless once it gels. Rather than simply using a slower-set resin and de-gassing, some casters use a faster-set resin and just skip the step.

Sometimes you get away with it, sometimes you don't.

There ARE some lower-viscosity casting resins available that claim to be thin enough to release the mixing-induced bubbles to the surface prior to casting, making vacuum de-gassing unnecessary.

As Bill mentioned, pin holes in resin can happen due to improper mixing, which can stir air into the mixed, but uncured resin, but those generally are few and far between pinholes.  If there are a LOT of pinholes,then more than likely the caster wasn't careful to keep his resin tightly covered in the bottles/jugs--THAT allows any moisture in the surrounding air to get into the resin itself (not the catalyst), which is fairly hygroscopic (meaning it readily attracts moisture)--that can actually make the mixed resin literally "foam" up if it's severe.

The best resin-casters use a pressure pot or tank which can safely hold up to 75-100psi, as air compressed to that extent has the ability to crush any tiny bubbles down to insignicance (as those got stirred into the resin at the local atmospheric pressure, and once the casting is cured hard, those tiny bubbles will not return.

The best way to fix pin holes that I found is to take a fresh #11 Xacto blade, and "twirl" it's sharp tip into the offending pin hole enough to just open it up--then fill the hole with a drop of gap-filling CA, followed by one of the various liquid accelerators.  Once hard (which only takes a minute or so at the maximum) the CA "patches" can be sanded smooth--they will be about the same hardness of the surrounding polyurethane resin.

Over a 12 year period, I produced hundreds of thousands of resin parts of all sizes--I think I learned a good bit doing it.

Art

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First many thanks for all the feedback on my pinhole issue. There has been tons of good advise put on the table.

Here is a statics update:

The piece I'm wrestling with is a resin cast hood. Presently, it`s in tthree coats grey primer. Two thin dustings of Tamiya and a third coat is a medium pass of grey Krylon sandable spot filler primer.  it looks good at first glance but when you get it under  light and closely eyeball the situation you can see several patches of 1/32 inch dimples here and there. These dimples are air bubbles that are right at the surface but haven't opened up yet.

In response to Astroracer advise.."I will not recommend spraying these parts with a "hot" paint, such as laquer. It WILL raise the pinholes through the primer."

My intent was to replicate an era correct  Baldwin Motion car in era correct factory colors so my color coat (which I have already purchased") was  going to be lacquer. 

I am open to all additional guidance at this point.

If you are seeing dimples in the casting,  not at all likely those are from air bubbles, the most likely cause could be that the caster did not use a vacuum pump and chamber to draw all the air bubbles out of the rubber itself after mixing, but before pouring his molds.  When this happens, the trapped air bubbles will expand if they are close to the surface of the master, as RTV rubber shinks ever so slightly as it cures, but those little air bubbles were mixed in at atmospheric pressure, so when the "hard support" of the nearby hard master is removed, the trapped air can and will expand, rasing a small "bump" on the inside surface of the mold--that translates into a dimple or small dent in the surface of the casting. Tthose "dimples" or small "depressions" in the surface (they look sorta like 1/25 scale hail damage, correct?  If so, they can be filled with a touch of putty over the primer, and sanded smooth (but it's a PIA, I know that).

The best resin casters, such as Modelhaus, or Detroit Resin Auto Group (there are others at their level as well) have invested some $$ in a proper vacuum pump and vacuum chambers which can pull mechanically perfect vacuums in mere minutes, which will draw literally all trapped air from the still liquid rubber--unfortunately many small, perhaps neophyte resin casters don't often go that far.

Art

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Those "dimples" or small "depressions" in the surface (they look sorta like 1/25 scale hail damage, correct?  If so, they can be filled with a touch of putty over the primer, and sanded smooth (but it's a PIA, I know that).

Art

Art you so nailed it..you must have been looking over my shoulder last night. Your description is sooooo on the money. At first I had a number of pin holes that I managed to have gotten read of; now I'm felt with this several clusters of 1/25 scale hail damage like dimples. This is so testing my patience.

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I respectfully suggest you open up one of the "dimples", if it will open.

Besides being flaws in the mold transferred to the part, as Art suggests, they may also be bubble-flaws in the part itself.

In my own experience, "dimples" are sometimes depressions in the part-surface caused by underlying bubbles, where there's nothing to support the very thin surface membrane.

The only fix is to dig them out, one by one, and fill.

 

Edited by Ace-Garageguy
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....they can be filled with a touch of putty over the primer, and sanded smooth (but it's a PIA, I know that).

The best resin casters, such as Modelhaus, or Detroit Resin Auto Group (there are others at their level as well) have invested some $$ in a proper vacuum pump and vacuum chambers which can pull mechanically perfect vacuums in mere minutes, which will draw literally all trapped air from the still liquid rubber--unfortunately many small, perhaps neophyte resin casters don't often go that far.

Art

Just a followup on the pin hole/ dimples issue I had with my resin hood. After five nights and one Saturday afternoon of tediously working on it (sand/ rinse/touch up prime/wait for it to dry/ access the results...sand/ rinse/touch up prime/wait for it to dry/ access the results..sand/ rinse/touch up prime/wait for it to dry/ access the results..blah..blah) I finally got the part to look 99% better.  I want to take a moment to thank everyone who took the time to give advise and assistance with the issues I was having with the resin part.

Art, truer words have never been spoken when you stated "it can be a PIA".  It was absolutely a test of my patience and resolve as well as a genuine PIA...LOL!!!  When you take into account the amount of hobby hours invested into making this one part useable for my project its a shame. I refuse to publicly bad mouth the caster from whom I purchased the part, its not my style. Overall, I view this challenge as a learning experience and in some twisted way wish to thank the resin caster for giving me this challenge so I could broaden my modeling skill sets. However, that said I assure you I will be not be purchasing any additional parts from them...LOL!   

Additionally, this challenge has afforded me more insight about the resin casting process, largely due to the knowledge many of you seasoned forum members have dropped on me...thank you.

It is said "any landing that you can walk away from is a good one"...."what doesn't kill you makes you stronger"...."the best knowledge is hard earned knowledge"...."live and learn"...."either your glass is half empty or half full"...etc. So, at the end of the day I look at this experience as a good thing. Many, many, many thanks to all those individuals who contributed in helping me overcome this modeling challenge. 

Best to you all,

69NovaYenko

Edited by 69NovaYenko
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