Polyester casting resin

polyester casting resin
5 things you need to know before using polyester casting resin

In my upcoming list of projects, I have a few ideas on the design table that involve using polyester casting resin.  Before I get into polyester resin tutorials though, I want to talk about several things you need to know before you use polyester casting resin.

1. The smell is horrible.  Unfortunately, this fact is not clearly stated on polyester casting resin containers.  I admit that my nose might be a bit sensitive to smells, but I have read the same comments from other resin casters as well.  For me, I will NEVER use this resin inside my house or office.  I will use it inside an enclosed space (like a garage) if I can leave the space, close the doors, and not reenter for twelve hours or more.  When working with this resin, I will use a NIOSH approved respirator for fumes.  *Note:  these are not required according to the safety information of the polyester resins I use, but I find the smell bothers me that much.  If didn’t have a NIOSH approved respirator, I would only use this resin outside.  In addition to that, I would have a fan blowing fresh air on me or evacuating air away from the casting.  Yes, I realize this is overkill, but at least to me, the smell is that bad.

2. This resin is not for beginners.  It requires more skill in several respects.  First, the pot time is shorter than many epoxies, generally 8 minutes or less.  Note:  The polyester resins I have used state their pot time is not that short.  Maybe it is because I’m in a warm weather climate, but none of the polyester casting resins I have used have ever gone more than 10 minutes before starting to gel.  To make things even harder, polyester casting resin does not mix in a 1:1 or 2:1 formula, but instead requires drops of hardener per ounce of resin.  If you are struggling to get a good mix with larger volumes of hardener mixed into the resin, you are not going to be successful only mixing in a drop or two of hardener into the resin.

3. You have to start with the end in mind.  While you can mix a volume of epoxy resin and hardener and use as you please, polyester casting resin is not that simple.  The amount of hardener you add to polyester casting resin is dependent on the depth of the final casting.  It seems counterintuitive, but the thinner the casting, the more hardener is needed for curing to occur.  This is because thin layers are unlikely to generate enough heat to cure.  You will need extra hardener to get curing to take place.  To take that one step further, when pouring multiple layers of polyester casting resin, each layer has an additive heat effect.  What this means for you is that subsequent layers likely need a bit less hardener if you are casting onto a layer that hasn’t fully cured yet.  What happens if you do not reduce the amount of hardener?  The resin can heat up to quickly and cause cracking around your embedments.

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4. The shelf life on polyester casting resin is no more than 6 months.  While many epoxies will work for years if properly stored (although the hardener may start to yellow), 6 months is generally the longest you can expect polyester casting resin to stay in usable condition.  Note:  In troubleshooting problems for readers who purchased their polyester casting resin from a ‘big box store’, oftentimes, I find the resin was likely purchased after the 6 month shelf life.  I make sure Resin Obsession rotates stock regularly to give customers the longest shelf life possible for all of our products, not just polyester casting resin.

5. The surface exposed to air during curing will remain sticky, even after the resin has cured.  You will either need to sand off this surface or seal it with a layer of resin gloss sealer spray to cover the stickiness.

So with all these disadvantages to using polyester casting resin, why should you consider it?

Polyester resin castings cure very hard.  They cure hard enough that you can use a buffing wheel and compound to polish polyester resin castings.  That means you do not have to recoat with another layer of resin or a gloss spray to get a shiny surface.  A few seconds on a buffing wheel is all it takes to get a bright, shiny finish.  That being said, however, if a polyester casting is dropped on a hard surface, it is likely to break.

Polyester resin is also generally ‘moisture insensitive’.  Your environment’s humidity generally does not affect its performance.

What has been your experience with polyester casting resin?


Unpublished Blog Posts of Resin Obsession, LLC © 2016 Resin Obsession, LLC



Cheryl Levesque

That is the polyester resin I used and almost burnt my house down about 2 hours after I poured it

Kate Rijacki Ledum

For the hardness factor, I would like to try Polyester resin at some time, but.. that time is not now and likely not until I have retired from the day job. I wouldn’t be able to consistently schedule time to do projects for it with only weekends available. Plus, I want to make sure I have an environment where I am comfortable working outside with it. But, oh, the thought of being able to use a buffer to shine it…


I am exhausted just reading your caveats. I think I am going to pass on that and stick to epoxy LOL

Katherine Swift

It can be exhausting! This all did sound scary didn’t it? I don’t want to scare you away, but want to make sure you have clear expectations on what might happen. (or what could go wrong 🙁 )


Polyester resin was the first I tried. Before that I didn’t know that there is difference in resins.
I’d like to mention one more problem. On youtobe.com (as usually 🙂 ) I watched as people suggested to use plastic yogurt trays for mixing components. I decided that it’s good idea! What happened? Resin melted the bottom of that yogurt tray…I was lucky that didn’t do this inside my home!!!!
Thanks for mentioning these problems in article! I good to know i am not alone 😀 After my first failure (when my “masterpiece” remained with sticky surface) I thought I did something wrong. And I did something wrong. It was said in instruction that in order to prevent sticky surfaces, it has to be covered so there wouldn’t be any air in between. Second time I tried to cover it by plastic , but that didn’t help either. The surface remained sticky and thanks to plastic- rough. And that terrible, terrible, terrible smell that don’t disappear for couple of days…

Mary Kay

I’m new at this, but would love do it. I have some beautiful old windows that I would like to put in sea glass. How heavy is this stuff? I would be working outside with it. Sound like the way to go with sea glass and window. Seen them in Pinterest.

Katherine Swift

Hmm. If you put it in thick layers, yes, I would expect it to be heavy. Were you hoping to replace the windows with resin or use the resin on top the glass already in the windows?


After years of using epoxy resin I tried polyester earlier this month. Smell is bad, smells like rubber cement x 100. Much harder to work with and goes right from very liquid to gel, unlike epoxy that gets more syrupy with time. But I have to say my deep castings for cabochons ended up so beautiful and clear, not a bubble in sight. For deep castings polyester is definitely the way to go. For anything else, stick with epoxy. In my humble opinion.


I have actually just started working with resin period and not knowing much about the difference I chose the polyester. I had a few trial an errors, but I so far I think I found that using about 10 drops of catalyst per ounce no matter what I am making has somehow worked out everytime for me. Yes it stinks and does gel up quickly, but it seems to turn out beautiful for me everytime now that I have worked out the bugs.


Same here. Only tried polyester, and non experience with epoxy. My pieces turn out beautiful every time. Just a few drops of hardener will cure each project. Curing time varies between 1 hour and 5 days. Just a few drops in a large batch of resin will take longer to cure, but all airbubbles will be gone. The end product is hard, and hardly any scratches over time. But indeed the smell is bad. True!


I have a question, do you know if this is safe for long skin exposure? I wrote envirotex lite and asked about making gauges for eats and they advised me it would not be save for long skin exposure. Would this type of resin be?


I’m having a rough (excuse the pun) time getting a clean finish. I didn’t know about the different resins and encased some sentimental flowers in a bracelet. It’s perfectly dry not cloudy however I tried to sand then buff the seams on the bracket and now I can’t get it to go clear again!!! It’s a cloudy scratched mess no matter how much I buff. How can I fix this?!

Brandi Jasmine

I have good results with a product by Mod Podge called “Dimensional Magic”. After you buff or sand, paint a thin layer of DM on your piece, and you should have a nice, shiny surface. It’s quite hard, too. You can also use it to “dome” surfaces. If you like a thick dome, do it in layers, because if you drop too thick a layer, it will dry cloudy.


I’ve been using polyester for a few months now and after lots of trials and errors I think I’ve cracked it! I added a slightly bit more hardener and I’ve found I have no sticky layer! Yay. Definitely makes the finished result more brittle but it’s fine for smaller pieces like pendants and it’s soo crystal clear


A quick warning about Polyester Resin. Its much harsher then Epoxy. Polyester will eat through plastic cups so you have to be careful what you used to mix it in. It can also take the finish right off some of your embedded objects and change the colour of some pigments.
It does dry super clear and hard. I personally love using it and its cheaper the epoxy too! But it does take a bit of prior knowledge of what it could react to before you find it just ate your favorite embellishment.

Poppy Daniell

Hello, I am working on a shoe design and I really need to make a pair of clear high heels, what kind of resin should I use to make the heel part, keeping in mind that needs to be resistant to weight?

A. Anderson

Hello, I have a question about stability. I cast a resin panel (approx. 5/8″ thick) with some rock embedments for a client and was just called back (after 4 years) to see that is has noticeably warped. It’s approx. 24″ a 30″, displayed in a wall opening (like a window) and over time has become convex – deflecting somewhere between 1/2″ and 3/4″ over 24″. Any ideas what is causing this and anything I can do to repair and/or arrest the movement? Love to hear your thoughts.

Katherine Swift

Hmm. I don’t have a good answer for you on why this happened. Unfortunately, there isn’t going to be an easy way to fix it. You can try to cast another layer on the front and back to even it out, but without some support on the sides, this will likely happen again.


What a dilemma: A couple of years ago I was given 2 large tins of XOR Crystal Polyester Resin and hardeners etc. and, because I did not want to waste any resin, I thought I’d wait until I had a good idea as to what to use it for. Now, after coming across your excellent website, I decided to give it a go; The resin inside the tins seems very hard, but clear, with a few walnut size snow-white crystals on top…
I guess I can not make it liquid anymore, so, hopefully it is rock-hard all the way through, then I can at least put it on the milling machine and make some cubes or whatever shapes out of it.
Sadly, I didn’t know that that stuff doesn’t keep….


Thanks for the info. Few questions. 2nd attempt. :/ using epoxy for 6 months and my 3D molds get Sri bubble holes in the finish product at top. Even in pressure pot. My molds are 80+ml (1-2”deep). My epoxy is fine w anything under 1” but after that the craters develops. Annoying. I’ve tried so much. Would you suggest any other type of epoxy or just this polyester type? Working outside isn’t really an option down here in sofla. High heat and humidity. Also. I guess w the quick set I wouldn’t need to worry on sealing the edges of the mold but would. I still be able to drop inks in? If it’s setting in 10 min Is that when it’s ready to demold? I didn’t see the demold time. Thank you.


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