Five things not to do when casting resin

Five things not to do when casting resinOne of the best ways to learn about resin is to push it to its limit.  Put another way, you want to push it to fail.

Why?  I can easily say I’ve learned more from my resin mistakes than any of my successes.   Being afraid of not making mistakes is where that comes from.  Of course, mistakes will happen.

I admit it.  I didn’t follow basic resin rules and made several blunders when trying to test using Resin Obsession pigments in polyester casting resin.  While I was able to get the answer I was looking for (Yes, they do color polyester resin!), the finished castings are nothing short of a big hot mess.

Mistake #1:  Try to cast resin when you don’t give yourself enough time.

polyester resin colors

Since I was working with polyester resin, I wanted to do this outside because the smell is so bad.  (By the way, if you have never worked with polyester casting resin, here are five things you need to know first.)  It was about 4 PM and I knew I would only have about another 60 to 90 minutes of daylight (and warmth) to work the resin.  I prepped my individual 1-ounce mixing cups with a toothpick of color from each of the twenty-five Resin Obsession color pigments.


polyester casting resin

I wanted to have enough polyester resin to put in 25 cups, so I poured as much of the resin as I could into one of my 10-ounce mixing cups.

Mistake #2:  Mix more resin than you can possibly use in the pot time.

adding catalyst to resin

While it was in the upper 70’s temperature when I started this, I knew it was not going to last as the sun went down.  I figured I would pour approximately 1/3 inch of resin into each cup.  Note:  Polyester resin is different from many other resins in that the amount of catalyst you add is dependent on the thickness of the casting.  According to the directions, I should have added 6 to 8 drops of hardener to this resin.  I wanted this stuff to get warm quickly to start curing quickly before the ambient temperature cooled off.

Mistake #3:  Add more hardener to get things to cure quickly and think there won’t be consequences.

casting polyester resin

I quickly poured resin in all of my cups.  This took probably two to three minutes.

mixing resin color

I stirred with the toothpicks already in the cup instead of getting stir stix.  The stir stix would have been a better choice here as it would have blended the color into the resin faster and with fewer bubbles.

Mistake #4:  Be too lazy or unorganized to get the right tools for the project.


bubbles in colored resin

So I didn’t get too far into this to realize that my resin was starting to gel.  By the end, I was mixing this like I was making scrambled eggs.  I was introducing too many bubbles.  I hoped I could get them out once I finished mixing.

Mistake #5:  Use poor resin techniques and think you can fix it later.

At this point, I had to get my assistant who was taking pictures to get gloves and help mix the colors.  Even with that, we couldn’t get to the end before I was dealing with a goopy mess.


polyester resin coloring agents

The good news in all this is that the resin did cure with the colors staying true.

polyester resin starting to gel

This was one of the final cups of resin that I got to mix before it started to gel.  (I was only about halfway through mixing the twenty-five cups at this point.)  Mixing the color into the polyester resin was like trying to mix the color into Jell-O.

By the way, the cup of resin shown here is fully cured.  Yes, it is lumpy and those bubbles are forever trapped in the mess I made.

opaque colors polyester resin

I was happy to see the colors cast as I expected they would.

These were some of the first cups of resin and color I was able to get mixed well.  You can see the holes the bubbles left behind on the tops of the resin.  While the bubbles were able to rise to the top, the resin was started to thicken and couldn’t level out after they popped.

neon colors polyester casting resin

So while they only managed to get swirls in the polyester resin before they cured, I think the Resin Obsession bright neon pigments are my favorite.  I think these have the potential to make great polyester resin coloring agents.



Wondering what else not to do when casting resin?

Then you will want to get a copy of my ebook, Resin Fundamentals.  I’ve condensed my fourteen years of resin experience into a easy to read book on the important details you need to know to be successful with resin on day one!  Buy the book now and you get a download link in your email inbox in minutes.

Unpublished Blog Posts of Resin Obsession, LLC © 2021 Resin Obsession, LLC

Like this post? You may be interested in  9 Facts Everyone Should Know About Resin Safety

10 thoughts on “Five things not to do when casting resin

  1. I am on the floor laughing here because I know each and every of these mistakes. Not with polyester resin but it really doesn’t matter. It happens with epoxy too. Pouring with not enough time planned will be a disaster. Oh and how many times have I mixed too much resin, poured some pieces, got a new idea and raced around half an hour in my apartment to look for the sea shells or glitter or whatever that of course have dissappeared? Just to find the resin with the stir stick in the cup gooey and just ready for the bin. And each time I swear it won’ happen to me again….

  2. Loved this and I like your humility. i jist recently started working with polyester resin in wood and am very pleased to learn that you color pigments will work well. I will definitely be trying it soon. Thank you.

      1. Hi

        I’m about to experiment colouring some water based casting clear resin.

        I use a product called “Brusho” which is a water reactive pigment.

        Have you or any others resinists used or had any experiences with this product?

  3. Oh goodness! Ok.. It is actually cooler here today than typical but I thought, sure set them out in the sun and they will cure… Haha.. I was tricked! Drats! Sooooo I used some acrylic paint to color mine.. Hmm still liquidity.. I think I am gonna try to make an oven with a clear top and foil sides and see if that helps.. Most I might have for a problem is condensation.. But I will try..
    I do typically use powdered make up, scrapbooking flocking, glitters and have even used soap dyes.. Never a problem.. But typically… The weather is hotter. I’ll let you know how it goes.

  4. In my experience the translucent dyes burn off in polyester, for instance I used the tiny bits of trans red everyday before I switched to polyester in Alumilite & it turned out blood red like I wanted (FANTASTICALLY GREAT!) but used the same dye & same amount even a great deal more/less & the end results were browns or purples depending how much coloring used. I use Silpak Polyester (SPR-41-F) 5gal jugs now so I’m casting a great deal everyday & most of these translucent dyes don’t do what they would do in the Alumilite resins. Cast a little piece in polyester & in Alumilite you will see a big difference in the colors (make sure they are mixed properly tho lol). Maybe the polyester I have is different than the cast & craft stuff idk. Any help is appreciated on what to do or where to find a new dye for polyester other than the cast & craft brand, it takes a half bottle of dye to do what I want its crazy. Need something more concentrated than the water they give you lol.

    1. The colors could add to the volume of the resin. You need to account for that with the amount of catalyst you use.

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