HOW TO USE EPOXY RESIN: TRICKS AND GENERAL
Definition: Epoxy resin is a two-component thermo-hardening liquid polymer.
Apart from the “technical” definitions, epoxy resin is a liquid product which, if exposed to a minimum temperature of 10-15°C [50-59 F], begins to catalyse and then solidifies.
Here are some “fundamental” points for who approaches this product for the first time.
How do you dose resin?
Since it is a two-component product, the package features 2 containers, one with resin “A” and the other with hardener “B”.
Equip yourself with an electronic scale to weigh the grams and with a container to mix the liquids
- Put the amount of A you think you need in the container, trying to get a round number in grams (e.g. 200 gr [0,44 lb]), this will make it easier to count how much hardener to add;
- Each product features on the container of B (or even on both) the usage ratio;
- Let’s make an example with 100A: 60B. Multiply the grams of A previously weighed (200g [0,44 lb]) by 60 and then divide by 100, the result will indicate the grams of B to be added.
Practical example (ratio 100: 60)
40 gr [0,08 lb] component A. how much component B should I add?
Solution A 40/100 = 0.40 A 0.40×60 = 24. This means 24 gr [0,05 lb] of B.
This formula can be used for any usage ratio, 100:50, 100:30, etc.
How to prepare the resin?
After having been accurately measured, the resin must be mixed in an irregular way (so, not always in the same direction) for at least 2-3 minutes. This passage is fundamental, since if you mix it only for a few seconds, the resin won’t be well homogeneous at the molecular level, and therefore it will remain soft/opaque in some parts
How much does the resin harden?
Hardness depends on the formulation, the resting time and the catalysis temperature.
Furthermore, the mechanical resistance of the resin continues to increase up to 4-5 days after application.
So, after 24 hours it may still seem soft, but in the following days it will continue to harden until it becomes like “hard” plastic (let’s say “plexiglass-like”)
How long does it take for epoxy resin to solidify? Is it possible to accelerate hardening?
Also in this case, it depends on the formulation and on the temperature.
However, since it is a “THERMO-HARDENING” polymer, the more it is exposed to heat, the faster catalysis is.
As a general rule, every 10°C [50 F] more, the catalysis time is reduced by half.
20°C [68 F] = 3h00’
30° C [86 F] = 1h30’
40°C [104 F] = 0h45’
To accelerate the catalysis, it is therefore sufficient to keep the casting close to a heat source (even a simple radiator). Be careful, however, not to heat up castings over 1 cm [0,39”] thickness. Resin may in fact heat up too much due to the mass effect (which is explained in the guide)
This term is used to describe the exothermic phenomenon (i.e. heat release) that occurs when the resin is poured in high thicknesses (greater than 1 cm [0,39”]). The A molecules, indeed, release heat when they bind with the B molecules, and it is immediately which is immediately dispersed in minimum thicknesses. When the casting thickness is instead higher than 1 cm [0,39”], the heat is dissipated more slowly, so the casting starts to heat up.
This heating further accelerates the reaction, which causes the temperature to rise even more. For this reason, it is necessary to avoid pouring too much resin all at once, in order to prevent it from “cooking” (creating cracks, bubbles and darkening).
Does the resin yellow?
Any resin (even with “advertised” UV filters) sooner or later, if exposed to sunlight, will tend to change colour, turning towards an amber shade. It may take a few weeks or many years depending on exposure to sunlight (outdoor it will yellow sooner), on the thickness of the resin (the thicker it is, the more it will be visible) and the type of colouring. For example, if the resin is coloured in a “strong” tone such as red or black, the yellowing won’t be visible… while if it is white or pink, you will detect it sooner.
Resins usually feature reduced-yellowing characteristics and can therefore be used for manufactured products that should not change colour for several years (if stored indoor), such as paintings, tables or jewellery.
How to remove surface opacity or small ripples that appeared right after solidification?
These surface opacities (especially in the cold season) are the result of the effect of ambient humidity, which creates a patina on the surface of the casting. At first you won’t notice it, but as soon as it solidifies, you will see matt auras and even a crinkled film, in some points.
To avoid this, there are several strategies:
- Working in a low humidity (dehumidified) or warm environment;
- Heat up the 2 components before pouring them (for example by holding them over a radiator);
- Mix the resin with the hardener and apply it only when it starts to heat up (so that the reaction starts when it is still in the container). This operation is essential, but requires a little care.
Resin (according to the quantity you have prepared, because of the “mass” effect) may in fact need a few dozen minutes before heating up. It should be checked every 5 minutes and as soon as it reaches 40°C [104 F] (that is when it is warmer than our hand), it can be applied. In this way, a part of the molecules has already reacted and it is therefore less vulnerable to ambient humidity. But be careful not to wait too long before pouring, otherwise it may solidify in the container!
- It would be a good rule not to apply it in the evening or when it rains (as the ambient humidity increases);
- Do not apply on surfaces that still retain moisture such as, for example, wet concrete or undried wood.
How to polish resin?
There are several methods. Much depends on the size and regularity of the surface to be polished.
The larger a surface (or irregular, such as a jewel or a miniature), the better it is to use spray paint. Transparent polyurethane spray used for the furniture lacquering is the ideal. If it is not available, the transparent acrylic spray you can find at the hardware store is a very common fallback. However, 2-3 sprays coats at least are necessary for a good finish.
The method that surely grants the best aesthetic effect is manual polishing with abrasive paper (up to grain 1500) and then finish it with the classic “polish” (i.e. the polishing paste) used to polish the plastic of car headlights. This technique however requires experience and an orbital polishing machine, otherwise there were still be scratches and irregularities. Recommended only for professionals with proper equipment, and for not too large and fairly regular objects.
Another method is to apply an epoxy resin finish with a brush (or by casting), after sanding up to grain 400.
We suggest to wait for the resin to become more viscous (by letting it rest in the container after the preparation) and then to reach a consistency close to “honey”. At that point, you can start applying it with a brush (or by casting, if you want to achieve a mirror effect).
How do you remove air bubbles from resin castings?
Although degassers are required to 100% remove the air bubbles, there are some tricks that can help you minimise the appearance of bubbles in the artifact.
- mix the resin for a longer time but more carefully;
- when you pour the resin, try not to drop it from above, but pour as close as possible to the surface;
- after mixing the resin, let it rest for a few minutes, in order for the air bubbles to rise;
- once casted, move a heat source (heat gun or flame, NOT hair dryer) over it to burst the bubbles on the surface.
Once the resin has hardened, is it possible to sand, cut or pierce it?
Of course. Once well catalysed (24h-48h) depending on the catalysis temperature, resin can be worked just like normal hard plastic.
What can be used to colour resin?
Basically, with anything. Including powders, natural earths, metallic pigments. As long as the colouring agents are “dry” (for example powders or sand), there are no problems (provided they are perfectly dry).
Instead, when you want to add paste or liquid colours, you need to verify if they are compatible with epoxy resins.
If they are not compatible (for example lacquers or gouache), only a few drops can be added, since an excess dosage may affect the mechanics of the resin and its shine (for example, by making it squishier or stickier).
To summarise… the Three Golden Rules
As it is difficult to remember all the tips, we recommend the beginners to focus on 3 key points:
- Properly dosing and mixing with electronic scale and mixing for at least 2 minutes.
- Apply at a minimum temperature of 20°C [68 F], damp application environments (or surfaces) should be avoided (if you are not sure of the moisture, let the resin heat up before pouring it, as described at the beginning of this guide).
- Use a heat gun / flame to remove air bubbles on the surface.