Why Resin Yellows and What You Can Do to Avoid It

If you've been working with resin for a while, you might have noticed some of your earliest pieces just don't seem to have the clarity of your most recent pieces... they seem to be slightly discoloured or they've taken on a yellowish tinge.

As disappointing as it is, this is actually a normal phenomenon. 

It's a fact that all resins will yellow... eventually! 

For some resins, that can be as little as a few months. For other resins, it can be a number of years. 

You may have seen resin kits where one part of the resin looks slightly yellow.

The yellowing usually occurs in the hardener side first, due to the amines that are present. Without getting too technical, when they're exposed to oxygen (like when you open and close the bottle), they oxidise, or turn brown. That's why RESIN HAS A SHORT SHELF LIFE.

But just because your resin hardener has yellowed, doesn't mean you have to discard it. In most cases, it will still cure properly... it just won't be as clear as fresh resin.

So we know that resin yellows in the bottle...

But it's the yellowing that happens to your cured resin pieces that I want to talk about today.

You might be surprised to know that part of the reason your projects have discoloured is caused by the way you used the resin. 

Some of these practices actually ACCELERATE the yellowing process so it's important that you know what these things are because there are ways to avoid or lessen the effect of the yellowing in your projects.

WHY RESIN YELLOWS

Here are some of the top reasons why your resin projects look yellow and what makes it appear more noticeable (in no particular order!):

1. Heat build-up or exotherm
2. Exposure to UV light
3. Resin poured too thick or deep for the resin you have chosen (you must choose the right resin for the depth of your pour)
4. Using a sealant that isn't UV resistant to seal items that you put in resin
5. Using white colourant in the resin or pouring clear resin over a white surface
6. Over-torching the resin or overheating it with a heat gun to pop bubbles

1. Heat Build-up or Exotherm

When you mix resin and hardener together, the chemical reaction that happens creates heat (an exothermic reaction). Heat is essential for the resin to cure, but believe it or not, too much heat is a bad thing. Resin that heats up too quickly can turn yellow.

These are some of the things that can cause heat to build up rapidly:

• mixing larger volumes than the manufacturer recommends, 
• pouring into deep moulds, which concentrates the heat 
• high room temperatures. 

When any of these things happen, the resin cures too quickly and that can turn clear resin yellow as it cures, even if it was completely clear when you poured it. This trinket bowl was crystal clear when it was poured into the mould but it yellowed DURING curing.

The instructions in some resin kits tell you the maximum amount of resin to mix so that you don't run into these problems. 

If yours doesn't, contact the manufacturer to find out their recommendation.

2. Exposure to UV Light

UV LIGHT IS RESIN'S ENEMY! 

It breaks down the chemical molecules in cured resin and causes the resin to yellow. And once cured resin has begun to yellow, there's no stopping it, even if the resin is no longer exposed to UV.

So always keep your resin projects out of direct sunlight. Even fluorescent lights can have an effect on resin's ability to resist yellowing.

Beware of exposing your resin pieces to the sun - it can cause other damage too: it can cause the epoxy to break down and the surface can crack and become chalky, and you'll lose the lovely glossy finish that resin is known for. 

The good news is that there are epoxy resins available with UV inhibitors and HALS (hindered amine light stabiliser) that delay the yellowing of resin. For long term non-yellowing, choose a resin with at least ONE of these two features. Or even better, choose one with both, like these:

• Art Resin - perfect for artwork
• The Epoxy Store's Clearcast 7050 - for artwork and shallow moulds (up to 6mm or 1/4")
• Unity Resin Premium Epoxy - for artwork and shallow moulds (e.g. coasters)

These additives will delay or even prevent yellowing, so when you need resin that doesn't discolour over time, it's worth paying the extra for these resins.  

Bonus tip: Avoid premature yellowing of your resin hardener by storing it away from direct light in an area where it's not exposed to high temperatures. If your resin kit came in a box, keep it in the box for added protection.

3. Resin Poured Too Thick or Deep for the Resin You Have Chosen

When you pour a coating resin (one intended for art) into a mould, it won't be as crystal clear as it would be if it was poured on a surface. The build-up of depth actually gives the resin a yellowish tinge.

If you want to fill a mould with resin, it's essential that you choose a casting epoxy, not a coating epoxy. 

Casting epoxies are designed for thicker/deeper pours, like filling a mould. Read more about why you need to choose the right type of epoxy for your project here.

Check the bottle or the instructions to find out how deep you can pour with the resin you have chosen.

4. Using a Sealant That Isn't UV Resistant

When you place paper images or dried flowers in resin, it pays to seal them first so you don't get "wet" spots. 

But many of the common sealants YELLOW WITH AGE and even if you use a resin that stays clear, a yellowing sealant can make clear resin look like it's yellowed.

Choose a sealant that:

• dries clear
• is UV resistant
• is non-yellowing.

Tip: If you've already got an old, opened bottle of sealant/white craft glue, then check to see if there's any dried glue/sealant around the pouring spout. If there's any discolouration, i.e. if it hasn't dried clear, then avoid using that product to seal anything you want to put in resin.

5. Using White Colourant in the Resin or Pouring Clear Resin Over a White Surface

Using white colourant in resin is another instance you'll notice that the resin has yellowed. It's not necessarily the white pigment that has caused the resin to yellow, but areas of white resin will become a shade of cream (or yellow in severe cases). Yellowing is also more noticeable in clear resin that has been applied over a white surface.

Here's a great example of white resin that has yellowed with time (along with the timber!). 

Tip: Minimise the effect of yellowing resin by colouring it! If you avoid clear or white resin, you won't notice the resin yellowing. 

6. Over-Torching the Resin

One of the most effective ways to pop bubbles is to briefly pass a flame (or butane torch) over the resin once it's poured. 

And as incredibly satisfying as it is to watch the bubbles pop, a flame (or a heat gun) can cause other problems if too much heat is applied when trying to pop stubborn bubbles. 

Concentrating the flame/heat in one spot too long can cause the resin to smoke (very dangerous). 

And it's another contributor to yellowed resin. Applying excessive heat can scorch the resin in those areas, causing spots to yellow and discolour. 

The trick with using butane torches, BBQ lighters and heat guns is to pass them BRIEFLY across the surface and not linger in one spot.

So remember, you can control a lot of the conditions that cause resin to yellow. 

And don't forget my BONUS TIP:

 

Store your resin in the box so that it's not exposed to UV light!

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Happy Resining!


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