Polyester casting resin – 5 things you need to know

polyester resin casting tips
Five things you need to know before using polyester casting resin

1. The smell is horrible.

I admit my nose is 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 leave for twelve hours or more.  When working with this resin, I 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 I didn’t have a NIOSH approved respirator, I would only use this resin outside.  I would also 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.

If you want to get a respirator, this article details the one I use:  resin safety precautions

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.

For beginners, I recommend starting with epoxy resin.  They are easier to work with and can also get you great resin casting results.

3. You have to start with the end in mind.

While you can mix a volume of epoxy and use it as you please, polyester casting resin is not that simple.  The amount of hardener you add is dependent on the depth of the final casting.  It seems counterintuitive, but the thinner the casting, the more hardener is needed.  This is because thin layers are unlikely to generate enough heat to cure so it needs extra hardener.

Why is this important?  You can’t mix the resin thinking you can pour it in any mold you have.  You might end up with a resin casting that doesn’t cure or one where the resin heated up too quickly and cracked.

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), six months is generally the longest you can expect polyester casting resin to stay in usable condition.  Note:  In troubleshooting users 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.

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 © 2020 Resin Obsession, LLC


Like this post? You may be interested in  Casting resin in non resin molds - Using resin in non traditional molds

80 thoughts on “Polyester casting resin – 5 things you need to know

  1. 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…

    1. 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 🙁 )

  2. 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…

    1. 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.

      1. 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?

  3. 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.

  4. 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.

    1. 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!

  5. 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?

  6. 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?!

    1. 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.

      1. I want to cast my wedding bouquet (after several practice rounds). Because of the depth, should I use polyester resin? Which resin is best for “larger” projects? And where is the best place to buy this much of it?

        Thanks for all your amazing insights!

  7. 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

  8. 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.

    1. It will not eat through most clear plastics (ex., polypropylene). It will, however, eat through polystyrene, which is what most cheap white plastic cups are made of.

  9. 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?

    1. ….keep in mind that the heel needs to be resistant to weight….
      I’m pretty sure she’s referring to the ‘weight’ of someone wearing them… I’m curious too. What resin would be best for making a clear heel?? Or even an entire shoe 👠 ?? Glass slippers yipeee! Could both work?? I know they make acrylic heals that would be the same look. Also, this might be a stupid question but, is acrylic the same thing as resin??
      Thank you!

  10. 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.

        1. 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.

  11. 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….

  12. 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.

  13. hello, ive used this a lot as well for cabochans into pendant trays. Have you found that they get scuffed up on the surface? I make mine to sell and I find that when i travel with them, theres always several that are unsellable from being bumped around, thx.

    1. I can’t say that’s been my experience. Polyester resin should cure to the hardness of glass and not scratch easily.

  14. So my projects cured with a rippled finish? Any ideas why? How can i fix the finished pieces so they look like glass? FYI Project is coasters.

  15. I’m planning on casting roughly 4” x 6” (d x l) cylinders for long-term salt-water submersion under pressure. Would polyester resin be a good/the best material for that?

    More questions:
    2) My embeds are extremely heat sensitive. I’m presuming I will need to cast with a channel (~ 1”)1 to insert the embeds into the center and then backfill with a plug. The embeds will be enclosed in a glass or plastic container. What concerns might I have with this plan?

    3) If I go with the cast-embed-plug method in 2) above, will the plug bond chemically with the surrounding material? Will it be visible? I plan to add a bulge in the channel via casting or drilling to provide physical resistance to the plug popping out and an insulator between the plug and the embedded item. Do you foresee any problems with this plan?

    4) Is it possible to “swirl” color in the polyester resin so I end up with ribbons of color throughout my clear cylinder?

    5) Finally 😂, is it possible to embed graphics on a clear substrate (e.g., a printout) in the polyester resin? What do I need to consider re: substrate or ink? Alternately, how would I put a hologram in this?

    Thanks so much for your help. This place rocks!


    1. I’m afraid I don’t have any experience using polyester resin for constant salt water exposure. I don’t know how well it would hold up.

  16. I’m new to casting and have started with clear epoxy. Seems easy enough but the moulds I want to cast need to be <3mm thick. I've found that epoxy is not rigid at this thickness, but I'm wondering if polyester would be?

  17. I want to cast a ring dia 4: which will hold strand of MS wires, Main reason for mould is to hold these wires to make a wheel brush.
    what will be best choice?
    I am going to do this first time.Please help

  18. Why my polyester resin released gas-like smoke and fumes? was it normal or i did something incorrect? I used Pp corrugated sheet for molding (and it also heated).


  19. I am looking to make a few jewelry pieces. Earrings to be exact. I am well aquanted with epoxy, but am liking the clearness of the poly. What would you suggest for a clear, skin safe, not extremely fragile pair?

  20. I don’t think this is a fair assessment. I make various resin items, mostly jewelry, and have used both poly and epoxy. I started with epoxy like everyone suggested and I HATE it. It bubbles, its soft, it doesnt shine well, and it takes FOREVER to cure. Waiting 7 days to be able to turn a piece on the lathe is absurd…and then when it does get to the the lathe it fogs and bends. Epoxy is garbage and the finished products look cheap and don’t hold their integrity as they yellow and fade over time. Yes, poly requires the use of a catalyst. This is no big deal at all. As a matter of fact, you can use the catalyst to have a better control of your cure time. Imagine my amazement going from 7 days curing with epoxy to 1 hour with poly! What a huge improvement to piece turnaround! Yes it smells. It’s a chemical. Let’s not be dramatic. Yes cure time is far less than with epoxy. Prepare yourself first then pour. Dont mix up several ounces with no plans. None of these issues above are actually issues. Poly is far superior to epoxy in nearly every way when looking create professional looking pieces in a timely manner.

    1. Hi Mikki, what specifically about this article do you think isn’t fair or accurate? It sounds like you have a lot of experience using polyester resin. Please, tell us more.

  21. I just bpught this clear polyester castin resin, i read the chart and been looming online for another chart, but it doesnt tell me how much catalyst I need for ounces, just for how think you want the layer. I’m doing a 2inch sphere with a flower and it holds 6 ouncesbut dont k ow how many drops to use

    1. Hi Rache, if you can’t find that information with the casting kit, you will have to reach out to the manufacturer for that information. Unfortunately every kit is different.

    2. As mentioned by Katherine, check the label or manufacturer’s website, since most polyester resins come with some accelerant mixed in (not always the same amount).

      However, the vast majority of polyester resins require between 1% and 3% catalyst, depending on temperature (lower temperatures need more catalyst) and how fast you want them to cure (the faster they cure the more likely they’ll crack, so slower is generally preferable unless you’re in a hurry).

      A “drop” isn’t really a fixed amount (depends on the eyedropper), so get a graduated pipette and make a test with about 1.5% catalyst (i.e., if you’re mixing 180 ml – which is approximately 6 ounces – of resin, use about 2.7 ml of catalyst), unless you’re somewhere very hot, in which case use only 1% (1.8 ml).

      If that takes too long, try about 2% (3.6 ml, for 180 ml of resin), and then 2.5% (4.5 ml).

      You generally won’t need to go above 2.5% unless it’s very cold (and, if it is very cold, exposed surfaces will always remain sticky, so you’re better off letting it cure somewhere warmer, instead of adding more catalyst).

      If you can’t get a graduated eyedropper, assume 1 drop = approximately 5 ml (but really, using a graduated eyedropper makes things more controllable, and you can get disposable graduated eyedroppers for a couple of cents a piece).

  22. I have never used resins of any sort and now doubting that I even can for my project but thought I’d ask. Spray painted bullet casings glued to plywood and set in angle iron framed coffee table. Very small crevices between casings. Depth from wood to top of table just shy of an inch but from top of casings to top of table approx 1/8″. Is it even possible?

    1. Hi Michelle, you have a lot going on here!

      Resin can work for this project, but I think this situation is best suited for a one-on-one consulting call where you and I talk back and forth about what’s going on so I can give you the best advice possible. A 15-minute consultation call is $20. I do them by video so I can have a chance to see what you are working on.

      If you are interested, please send a message through the ‘contact us’ page and I can get this going for you.

      Katherine @ Resin Obsession

  23. I work with Dollhouse Miniatures. I would like to make “bottles” for Lotion, perfume, etc. using gelatin
    capsules and resin. I have been told to use Envirotex Lite. Is there something better? The largest are .75″ tall and .25″ wide. The smallest are .4″ high and .13 wide. Just think of the vitamin supplements you take!

    1. Hello! I have a very special resin project I want to do for my stepson. His mother just passed and I have some of her cremains and a dried rose from the funeral that I want to add to the Castin Craft Polyester resin in a 2.3 inch cube. I have been researching like crazy and want to be sure I do it perfectly for him. I will obviously practice first.

      I have 32 oz of the resin. I am confused because it says to add a drop of catalyst to each layer for single layer pours and the largest layer is 1 inch. Does that mean I have to pour the cube in 2 layers?? I am really confused.

      I also have a tiny piece of an index card that I printed where she had written “love” and I put modge podge over it. I am hoping to put it down as well as some powder pigment to make some swirls. Since you have more experience in this, do you think it may work?

      Thank you 🙂

      1. Yes, 1 inch is the deepest Castin’ craft recommends pouring the polyester resin. As for sealing the paper to put in resin, mod podge does work.

  24. Thank you! Does that mean I have to pour a 1 inch layer and wait and pour another? I am not sure how long to wait between layers.

  25. Ok, I just attempted my first one! However, how in the world do u clean your mixing cups?? Mine are silicone. I tried water from the hose outside and nope ..not happening. Any ideas?

  26. Hello! Did anyone ever tried using pressure pot for casting polyester resin? I’ve tried it, but the results make the surface of the resin to wrinkled and sticky, like some tree bark from alien planet , does anyone know the cause?

  27. I pouring something that is 9″ deep by 12″ , a round tube. There is an object inside. What is the best resin to use and can I pour the whole depth at once.

    1. Hi Ashley, you will need a deep-pour casting resin and stay within the recommended guidelines for the maximum amount to pour at once.

  28. Hi Katherine. Thank you very much for a very informative article. I have just recently started casting with polyester casting resin, doing a small little table with wood imbedded into the resin. came out really well to my surprise. with all the info i also found on your page, i am rather excited for my next project. thank you again.

  29. Hi, I’m thinking of making a mailbox POST out of polyester resin. It would need to be 45in high and 2 in diameter. 2 questions what can I use as a form and is this a feasible idea?

  30. HELLO ! Everyone Im doing a concrete table with a couple of rivers running thru it. My ? is . will it bond to concrete ?( concrete is cured completely and sealed on top only but not where the resin make contact ) Right now the table is all one pc. the river section vary from 1/2 in to 3/4 in depth and the width is anywhere from 1/2 to 2in and 2ft in length. Would I be able to use this productto make the rivers? Oh also Can it be mixed with mica powder:? Thank you

  31. Hello!
    I plan to cast polyhedral dice (each approx 1.5cm in height) using polyester resin. Considering the properties/durability of this variety of resin, what is the likelihood that my dice will shatter/chip when thrown against a tabletop?

    1. Hi Diva, I wouldn’t expect them to chip on a gameboard, but they might if they are dropped on a hard surface.

  32. I don’t know if this has been asked and answered.
    I’m brand new to resin, but a veteran nail tech. I’m very used to working quickly with products before they cure.
    For a nail art competition, I want to make a clear resin hand holding a cellphone.
    I’m thinking of casting in alginate.(silicone is sooo expensive $$$)
    I know epoxy resin doesn’t like the moisture in alginate, but I’m wondering if polyester resin would do OK.
    I don’t have a pressure pot, and am a little worried about bubbles.
    Any advice you can give would be greatly appreciated.

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