Monday, September 28, 2020

How to avoid Sticky or Soft Resin

Have you ever had a piece of resin that didn’t cure properly and it feels sticky to the touch?

Or is your cured resin soft and bendy?

Piece of transparent green textured resin being bent

You know something’s gone wrong, but you have no idea what it is or how to avoid it happening again.

Well, don’t be too hard on yourself… it happens to the best of us at some point. It can easily happen if you get distracted.

So let’s have a look at some of the many reasons why your resin stays sticky or doesn't harden properly.

1. Inaccurately Measuring Resin and Hardener

The biggest culprit is usually off-ratio measuring. That sounds like a bit of a fancy term but what it really means is that the resin and hardener weren’t measured in the exact ratio that the manufacturer specified. 

It’s important to measure the resin and hardener in the ratio recommended for your resin. Your resin kit will tell you if it’s a 1:1 or a 2:1 ratio (these are the two most common ratios for craft resins) and it will also say whether you should measure by volume or by weight.

Cups of resin and hardener measured equally in 1:1 ratio

To get perfectly cured resin, you need to measure it accurately. So use graduated measuring cups for volume or a digital precision scale for weight. With resin, close enough just isn’t good enough!

2. Not Mixing Resin Well Enough

The second most common reason for soft or sticky resin is not mixing your resin well enough

A good stirring technique is important to make sure that you get all the resin and hardener to combine. That doesn’t mean whipping it - in fact, that will just cause you bubbling issues, and nobody needs that! 

But it does mean stirring around and around, purposefully, as well as from side to side.
Scrape the walls and the bottom of the cup a couple of times during mixing to release unmixed resin too. Then stir that through as well! 

Keep mixing it until you don’t see any syrupy-looking striations in the resin. Your resin kit will have more detailed instructions so make sure you follow them to the letter.

Syrupy striations in resin being mixed in a cup with a wooden stir stick

Bonus Mixing tips:

  • Pour the resin into a second cup and mix it again. Some manufacturers even recommend this two-step mixing method!
  • Use a stirring stick with a flat end to make it easy to scrape the bottom of the cup and to get into the corners of the cup. It will lessen the chance of unmixed resin ending up in your project.

3. Old Resin

Resin has a relatively short shelf life of only 12 months from the date of manufacture, and once it’s opened, you only have about 6 months to use it. Older resin will often still cure just fine, but sometimes it doesn’t, and this can be another cause for why your resin didn’t cure properly. I always recommend that once you open a bottle, you use it up as quickly as you can. And only ever purchase as much resin as you can easily use up in 6 months.

4. Moisture and Humidity

Resin hates moisture, so working in very humid conditions or adding things to the resin that contain moisture will stop the resin from curing properly. Excessive humidity can also make the resin appear milky once it's cured too. Make sure that any colourants that you add are suitable for colouring resin.

Hygrometer reading at 80% relative humidity

5. Low Temperatures

Resin is fussy about temperature. When the room temperature is too hot, the resin will cure too quickly. But when the room temperature is too cold, it won’t generate enough heat to cure and it will stay sticky or not cure completely. Check the instructions in your resin kit for the ideal room temperature for your resin.

6. Using the Wrong Type of Resin

Resin formulations vary from brand to brand and they all have different characteristics. Some are hard and some are soft. If you choose a soft resin for a thin mould, like a bookmark, the resin will cure but the item will always remain flexible. You need a resin with a high Shore D rating like this one for shallow pours.

7. Mixing Batches of Resin that are Smaller than Recommended

For resin to cure, the chemical reaction must produce heat. Mixing tiny amounts is not recommended because it may not generate enough heat for the resin to cure. Some resin kits even tell you the smallest volume you should mix, so check the instructions to see if yours has a minimum. 

Accurate measuring is even more critical for small batches so that the resin cures properly. If you're working on small projects like jewellery bezels, choose a resin that is formulated for small batches like this one

8. Adding Too Much Colourant

When you’re colouring resin with dyes and pigments, add no more than 10% of the total resin volume. The less you add, the better. Because resin pigments and dyes are so concentrated, you won’t even need to add this much to get the colour you want.

If you go above this amount, the chemical balance will be upset and when the resin cures, it will be soft.

9. Using Incompatible Colourants

There are lots of colourants that will work in resin that aren’t designed for resin. But some of them will prevent the resin from curing properly. When you’re starting out, it’s best to stick with dyes and pigments that are designed specifically for resin. And above all, avoid any colourants that are water-based such as vegetable dyes.

4 vegetable dyes inside a red circle with a line through it

10. The Resin Needs More Time to Cure

Epoxy resin generally takes 24 hours to cure. If the resin hasn’t cured in 24 hours, some part of your process isn’t right (e.g. low temperatures can lengthen curing time). Sometimes, leaving it for longer will solve the problem. But if it hasn’t cured in 48 hours, it’s unlikely that it’s going to cure.

11. Your UV Resin Still Feels Tacky After Curing

Some of the cheap UV resins seem to stay tacky on the surface even though you’ve followed the instructions. Try one of these solutions:

  • Use a UV lamp with a stronger wattage;
  • Cure the piece under the lamp for a LOT LONGER than stated on the packaging; or
  • Place the piece in the sun to finish the curing process.
36 watt nail lamp on a pastel green, purple, pink and yellow background

These simple solutions will often fix tacky UV resin.

N.B. Some cheap UV resins will become sticky again many months down the track. For these pieces, clean them off with denatured alcohol and recoat them with a better quality UV doming resin like this one.

12. Using Two Different Resin Kits

Sometimes, you’ll get to the end of your resin kit and have more of either the resin or the hardener left. Avoid topping it up with resin or hardener from another kit, even if it’s the same brand. It’s most likely that the two kits have been manufactured at different times or stored under different conditions and mixing them together can give unreliable results. And sometimes, the manufacturer has even changed the formula, so the two kits are incompatible! 

13. Using Resin & Hardener from Two Different Manufacturers

Never mix resin and hardener from two different brands together because they will have different formulations and the results will be unpredictable.

And one last one, 

14. You've Used Polyester Resin

Curing with a tacky surface is one of the characteristics of polyester resin. It only occurs on the surface that’s exposed to air so if it’s in a mould, it will cure hard on the sides and bottom and only the top will have that tackiness. You can remove the sticky surface by hand sanding and polishing. Polyester polishes to a very high shine. 

Now you know how to troubleshoot sticky and soft resin. Bookmark this article so you can refer back to it the next time you run into trouble.

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Sheet of textured green resin being bent

Happy Resining!

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Would you like to comment?

  1. What is the best temperature making the resin? Thank you for the other information above.

    1. The ideal working temperature for resin varies a little from brand to brand. It should be noted in the instructions in your resin kit or on the packaging of your resin kit. If it isn't, contact the manufacturer and ask them.
      As a general guide, epoxies like the room temperature to be between 21-25°C (70-75°F).


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