Five Things NOT To Do When Casting With Resin

Five things not to do when casting resin

One of the best ways to learn about casting with resin is to push it to its limit. Said another way, push it to fail.

Why? I can easily say I’ve learned more from my resin mistakes than my successes. It’s a good thing I can learn from my mistakes because, boy, did I push the limits of polyester resin recently.  Way more than I should have.

I didn’t follow basic resin rules. I made several blunders testing Resin Obsession pigments with polyester casting resin. While I did 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. It was about 4 PM in January, and I knew I would only have another 60 to 90 minutes of daylight (and warmth) to work the resin. So I prepped 1-ounce mixing cups with a toothpick of color from each pigment.


I wanted enough polyester resin to put in 25 cups, so I poured as much of the resin as possible into a 10-ounce mixing cup.

What I should have done: Waited until I have double the time needed for a project. With plenty of time, this resin casting project should have taken an hour. That means I should have budgeted two hours to cast resin.

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


polyester casting resin

While it was in the upper 70’s temperature when I started this, I knew it would not last as the sun went down. I figured I would pour approximately 1/3 ounce of resin into each cup.

That meant I needed about 9 ounces of resin for this resin experiment. And I decided to mix all that resin at once.

Now, the pot time of polyester resin is 7 to 10 minutes. And here I thought I was going to mix, pour and color that resin twenty-five shades of amazingness in that time.

What I should have done: Mixed the resin in two to three batches instead of one big batch.

Mistake #3:  Add more hardener to speed up curing and think there won’t be consequences.

adding catalyst to polyester

According to the directions, I should have added 6 to 8 drops of hardener to this resin. But, I wanted it to warm quickly to start curing before the ambient temperature cooled down.

By the way, polyester resin is different from other resin types. The amount of catalyst you add depends on how thick you’re going to pour the resin.

What I should have done: Used the right amount of hardener and tried another way to get resin to cure quickly.

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

casting polyester resin

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

mixing colors


After pouring casting resin into the cups, I stirred with toothpicks instead of using stirring sticks. The stirring sticks would have worked better because they mix color faster and with fewer bubbles. But, I didn’t have them close by and didn’t want to waste pot time looking for them.

So I didn’t get too far into this to realize that my resin was turning thick. I was introducing too many bubbles and thought I could get them out once I finished mixing.

What I should have done: Gotten all my supplies together before mixing resin instead of using whatever I could find in the moment.

Mistake #5:  Thinking I could beat the clock on a resin’s pot time.

cups of colors in polyester

At this point, I had to ask my assistant, (that guy who promised to love me through eternity in situations like this), to help mix the colors. But, even with that, we couldn’t get all the resin colors mixed before we were dealing with a goopy mess.

Even having the extra hands to help, we couldn’t get the colors mixed in before it gelled.

polyester resin starting to gel

This was one of the last cups of resin I mixed before it started to gel. Mixing the color into the polyester resin was like trying to mix the color into Jell-O.

What I should have done: Not overestimated my abilities because the resin always wins.

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

opaque colors polyester resin

These were some of the first cups of resin and color I could 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 started to thicken. It couldn’t level out after they popped.

neon colors polyester resin

The Resin Obsession bright neon epoxy pigments are my favorite. I’m thrilled that these make vivid polyester resin colors.

Wondering what else not to do when casting resin?

Then you will want a copy of my ebook, Resin Fundamentals.  I’ve condensed my fifteen years of resin experience into an easy-to-read book. You’ll get the crucial 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 © 2022 Resin Obsession, LLC

Like this post? You may be interested in  THE Essential Tips For Working With Epoxy Resin

10 thoughts on “Five Things NOT To Do When Casting With 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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