Bubble-Free Resin Casts With Modified Paint Tank





Introduction: Bubble-Free Resin Casts With Modified Paint Tank

This instructable shows how to modify and use a pressure paint tank for pressure casting. This is mainly useful for achieving bubble-free transparent or translucent resin/urethane casting effects.

The first step involves modification of a low-cost paint tank and the following section depicts the pressure casting process. I assume you have a mold (here is a great instructable on making a mold and video on making a simple block mold like I'm using below) and have basic knowledge of casting.

A professional 5-gallon pressure chamber costs upwards of $700, not including the compressor. A modified paint tank should cost under $120, and of course you will still need an air compressor.

You will need:

  • Pressure Paint tank
  • Air compressor with Air hose
  • 1/4" Compressor Coupler
  • 1/4" Female Compressor Plug
  • 1/4" npt cap
  • 3/8" npsm (fine thread) cap
  • Thread Seal Tape
  • Hacksaw
  • Wrench

in addition to the mold (ideally free of air-bubbles), casting material, and a release agent.

Note: the gallery image shows two casts of the same material (Smooth On's popular 325 urethane plastic), one that was pressurized and another that was not. I used the exact same mold and process for each cast except for pressurizing the first cast.

Step 1: Set Up Compressor, Acquire a Pressure Tank, and Read Manuals

First things first, if you have never operated an air compressor, understand that it is a very noisy machine that can be dangerous, especially in combination with a pressure chamber. Read safety manuals before proceeding to make sure you understand how to safely operate the machine. If you are purchasing an air compressor just for use with the pressure tank, a good & relatively inexpensive one is "the pancake" (you'll need a starter kit too). Don't forget to drain it after each usage to ensure its maximum lifespan.

Once you have an air compressor with an air hose and attached 1/4" female quick-connect coupler you'll want to acquire a pressure paint tank. I've been using the 2 & 1/2 Gallon Central Pneumatic Pressure Paint Tank from Harbor Freight. The price is advertised as $100 online, but I bought mine from a local store during a holiday sale for $80. I don't think its too critical which brand you choose. Perhaps more important is the size to make sure it can accomodate your mold(s).

Step 2: Convert the Pressure Paint Tank Into a Pressure Chamber for Casting

Four things need to be done to convert the pressure paint tank into a pressure chamber for casting. For the first three steps that involve attaching a plug or connector, you will need to use thread seal tape. Make sure that all three of these plug/connectors are tightly screwed on.

NOTE: for steps 2 & 3, I recommend bringing your paint tank to a local hardware store and testing caps on the appropriate connectors/outlets to make absolutely sure you are purchasing the right parts.

(1) Wrap thread seal tap around connector to left side of regulator and attach a 1/4" female plug using wrench of your choice (I think its easier to use an adjustable wrench but this will most likely scratch the cap. I don't think this really matters because I don't plan on ever removing the cap, but if you're concerned about this, then use a constrictor strap wrench).

(2) Wrap thread seal tap around connector to right side of regulator and attach the 1/4" npt cap using wrench of your choice.

(3) Wrap thread seal tap around paint outlet and attach a 3/8" npsm fine thread cap (different threading than previous cap) using wrench of your choice.

(4) On the underside of the lid, there is a metal tube. Take a hacksaw and simply cut this off. In the conventional usage of the tank, this is the draw for the paint, but since we're closing off the paint outlet and using the chamber for something else entirely, we'll want to create maximum space inside of the chamber.

Step 3: Using the Pressurizer: Prepare Mold, Pour Material, Put in Chamber, and Pressurize

Now that the paint tank is converted into a bona fide pressure chamber, its time to prepare your mold for casting. I will be using Smooth-Cast 325 (this process is also essential for the Crystal Clear series as well), so there may be some steps that are specific to this material. Always read material data sheets and consult with technicians during your purchase for further details. If you need to brush up on mold-making terminology, here is a good resource.

Before we start pouring, I think its important to discuss other potential problems, as a lot can go wrong in the process and result in not only lost time, but serious money. First, if you are using silicone as your mold material, you should know that most RTV silicone kits need to be vacuum degassed in order to reduce surface air bubbles on the interior of the mold (here's a video on the process, note the poor pouring technique at 1:57 - in order to further reduce air bubbles, technicians at Smooth-On recommend that you pour high, not directly on top of your objects, and only in one spot as shown in this video at 1:12). Artifacts in the mold cannot be fixed by pressurizing.

Professional vacuum degassing chambers costs upwards of $200, but thankfully there is an Instructable on making a DIY vacuum apparatus for about $20. There are other tricks that people use to prevent air bubbles, such as putting the silicone in a large ziplock bag, mixing, cutting a corner off and pouring. I guess its worth a try, but if you're pouring a mold that needs more volume, I think the DIY vacuum apparatus is the way to go.

That all being said, there are a few products that de-air themselves, though they're not perfect. One such product is Mold Star, and of course you'll want to practice the techniques shown in the product video. I have not seen other products that de-air themselves but am sure they exist. Unfortunately there is no resin that pressurizes itself, hence the need for this Instructable.

Ok moving on. Once you have a mold ready to go and are working in a well-ventilated area, put on gloves, glasses, and a mask before spraying release in the mold. Let sit for about 15 minutes, or however long your release recommends. Mann's Ease Release 200 is the recommended complimentary product for Smooth-Cast 325.

Next, mix part B first (clearly labeled on product). Keep stir stick on bottom of container and be sure to scrape sides too. If you are adding dyes or any additives such as a UV resistant curative, Smooth-cast requires it to be mixed during this step.

After part B is thoroughly mixed, add an equal amount of part A (Smooth-cast 325 has an easy mixing ratio of 1:1). At this point you'll need to work fast, as the working time ("pot-life") of 325 is about 3 minutes. If this is too quick for your application, you can also try 326 or 327 which have working-times of 7 and 20 minutes, respectively. Be sure to have everything set up before adding part A (i.e. compressor and pressurizer ready to go). When part A is added, remember to mix gently as this material has a low viscosity and can splash around. Its also unnecessary to mix vigorously like you would with silicone. I mix for about 45 seconds, and always time myself with a stopwatch.

Once the material is mixed thoroughly, "pour low" and in one spot, which helps prevent the introduction of additional air bubbles (i.e. bring mixing container as close to the mold as possible, then pour and hold steady). After your pour, you should notice some air bubbles on the surface, but since you're about to pressurize the cast, you don't have to worry about them! (Note: pouring low is much more critical when NOT using a pressure chamber...in any case it never hurts to practice good technique).

Next, place the mold in the tank. I use a circular piece of wood to make sure the mold will not fall over, as the bottom of my paint tank is concave. Once the mold is inside be careful not to move the tank, as the mold could topple and spill (learn from my mistake and be conscious once mold is in tank). If it happens, don't try to clean up the material - wait until it cures and then pop it out. If you're worried about this happening, spray some release in the tank beforehand so clean up will be easy.

The following step is a recommended practice from the paint tank manual. The manufacturer recommends that the opposite lid screws be tightened at the same time when sealing the lid. I tighten these screws in tandem one full-turn at a time before alternating to the opposite pair, which ensures the lid is sealed equally at all points. If not sealed properly (i.e. too tight in one place or not enough in another spot), you will hear the sound of air leaking and the optimal pressure will not be reached. I think its wise to practice pressurizing the chamber once or twice before pouring a mold to double check that all caps and lid are secure.

Once lid is sealed tightly, take the end of the air hose, peal back cover and then attach to plug, as shown in images. Then turn on compressor and monitor pressure - the compressor will reach between 20-30 PSI before the paint tank's gauge moves. Once the paint tank's gauge reaches between 45-50 PSI, turn the compressor off. You should not hear air leaking, and the pressure should remain constant.

Leave the mold in the paint tank for the duration of its demold time, which is the earliest recommended time that a cast can be removed from a mold.

Step 4: Demold and Examine Cast

After the demold time is up, release the compression coupler from the plug. I recommend wearing ear plugs because it is LOUD. Then unscrew lid and pop cast out of mold.

While pressurizing helps achieve superior results, its the sum of all good practices & techniques that will ultimately produce the best work. My pressurized crystal looks great compared to the unpressurized cast, however, if you look closely, there are some artifacts from not degassing (I used Mold Star 15 and degassing is still recommended, which I didn't do) and pressurizing too high (above 50 PSI), which resulted in "the measles" - tiny holes that damaged the mold. Initially I followed the technical bulletin for the Reynolds Advanced Materials pressure chamber (which recommended pressurizing at 60 PSI) and later talked to technicians who advised me not to exceed 50 PSI. At that point, however, my mold was ruined. Hopefully yours will not be. Since this first experience I have not damaged other molds pressurizing at 50 PSI, and wanted to show what can happen if you exceed this threshold.

I wish you luck in your casting endeavors and feel free to post comments if you need additional advising. I will do my best to answer your questions though I am by no means an expert, and have had a lot of help along the way in my journey as an object fabricator.



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what about vibration? can't vibration cause the bubbles to move to the surface? i know with urethane you typically have a very limited pot life, but i was just wondering if anyone puts there filled molds on a "vibrating table" (i think there is name for these)???

I have tried it and the technique doesn't seem to work for resin casts and it actually causes bubbles in silicone (although it works well for plaster). I haven't seen any results close to casts that have been pressurized.

Another source of bubbles in the cast is the water from air. I try to do casts where bubbles could be critical over the fall/winter time when humidity goes down to below 45%, and preferably even lower than that. And of course a combination of vacuuming the components and pressurizing the cast is what works best - neither seem to solve the problem completely just by itself.

As far as vacuuming, I usually only vacuum part B, mostly because I often need to add color or mica and those are very potent sources of bubble, especially mica. If you're going for metallic look and need to add mica, there is no way you can cast anything without vacuuming your part B first - your cast will look like Spongebob Square Pants.

I could not grasp from other comments here, are there people here that vacuum the mix of A and B ? Some of the comment sound like this is what happens and I just cannot wrap my head around how's that possible unless you're using a 30+ minute resin like CrystalClear. I usually work with 5-10 min SmoothCast325 - seems to leave you no time to do both vac and pressure. Still, if someone does that on a quick curing mix, I'd love to know how!

I'm just getting started with resin casting and I'm one of those people who try and read everything before I actually get hands on, and I just wanted to say thank you for this tip as it seems like it will solve a headache before I even run into it!

yeah, I'm wondering too how people can vac & pressurize resin at same time, though it could be possible with 326 or 327.

as for the degassing, I've never heard of vacuuming only one part. sounds like your application is pretty specific though, but I still think that you'll induce more air bubbles if you mix by hand after degassing.

Part A is less viscous (seems to me, gotta check the tech sheet though) and bubbles seem to come up much faster than in part B.  I just shake the Part A can well in advance of casting and just leave it sit, bubbles seem to come up by themselves and are mostly gone in 30 mins or so. Part B can sit with bubbles for hours. I guess they will eventually come up, too, but it's just easier logistically to help them by vacuuming.

Anyhow, the main reason I vacuum Part B is because there's usually much work that needs to be done with it - mixing paints and sometimes mica - both of which leaves plenty of bubbles and mica in particular is extremely bubbly.  If you do need mica, you have to work quickly before it starts separating to the bottom, so you just cannot wait for the bubbles.

I guess you can say the application is pretty specific though - together with my wife we are casting parts of fantasy BJD art dolls (samples here)  and colors are very important. There are no parts that don't have at least one of two different paints and mica is used often for parts that need metallic or pearly shine, such as mermaid tails and such.

By the way +1 on vouching for Smooth-On, those guys are great, most especially in art casting / mold making, which is what I'm more familiar with. Their resins are very UV resistant, too - important for pieces that you're going to be displaying in bright light.


Cool stuff, thanks for sharing, it's always nice to see what others are doing with these materials.

Just for people who may not be aware, not all smooth on resins are uv resistant. Ex. Crystal clear is but 325 series is not. However there is a one part curative called "sun devil" that can make 325 uv resistant.

Hey guys I have got a revolutionary vacuum/pressure tank that you got to check out


where did you get the mold? I need that so bad!

If you can't find or afford the crystal you want, you could also make it using plaster. Pour a block, rough-cut it with a junky saw, finish the flats by wet-sanding with sandpaper glued to a board. (you'll go through some sandpaper doing it, which is one reason why you probably don't want to use a sander with its expensive belts) You can fill any holes with more plaster. Wash all the dust off, let it dry completely, wax it good with paste wax and buff. You're good to go, and the wax should act well as a release.