Using resins is becomming increasingly common for DIY, GRP Flat Roofing, Car Repairs, Boat Repairs, Swimming Pool Restorations and Product Custom Mouldings.

But which one should you use: Polyester or epoxy resin? We have compiled some of our top tips starting with four questions you should ask yourself to determine which resin suits your needs best.

What material will the resin to be applied to?

This is a really important question if you want to achieve a perfect chemical bond. It is important to know the material of the product that you are repairing, to help choose the correct resin to ensure a strong permanent repair.

Polyester resins have a strong chemical bonding with other polyester parts and most thermoset plastics. It can also adhere to wood and some metals (ensure metal is scored before applying resin). It is important to note that polyester resins contain styrene that could dissolve polystyrene material and polypropylene.

Epoxy resin forms stronger bonds when it comes to gluing two pieces together and has better adhesive properties with surfaces like wood, metal or concrete. If the repair is critical or structural, an epoxy resin should be considered. If you are repairing an epoxy product, then epoxy should be used for the repair (polyester will perform poorly in this case).

If you are repairing a polyester product, or non-critical part you should use a polyester resin. If your application is gluing, load bearing or repairing an epoxy part, then an epoxy resin should be considered. In any case, the surface that you are repairing will need to be scored before applying the resin. It should also be clean and free from impurities.

What reinforcement can resin be used with?

The main reinforcement materials used are Glass mat (fibreglass), Aramid (often called Kevlar) and Carbon fibre. Fibreglass can come in the form of chopped strand mat, surface tissue and woven roving fabric. Carbon fibre and Kevlar come in fabric with different woven patterns.

Generally, you can associate any reinforcement material with polyester resin.

Epoxy tends to be used with either Aramid or Carbon, but can be used with fibreglass. It is important to keep in mind that if you are using fibreglass mat with epoxy resin, it has to be a powder bound mat. Emulsion bound mat contains styrene only dissolved by polyester resin, so it shouldn’t be used with epoxy.

Chopped mat should be used for general repairs, while woven glass, carbon fibre and Kevlar should be used when extra strength is required. When using woven roving, a layer of chopped strand mat should be used as the final layer to prevent peeling.

If you are using fibreglass mat with epoxy, make sure it is powder bound, there is no constraint using polyester or epoxy for other reinforcement materials. Always ensure that the correct amount of resin is applied to the mat. If you are unsure, please use our materials calculator which will give you an indication of how much resin you need per square metre of matting.

Calculate Fibreglass Materials

How much working time do I need?

Resins react differently depending on the environment. The temperature is one of the key parameters, as higher temperatures will cause resins to cure faster. This is why working with resins should always take place between 15 to 25°C. The working time of any resin is manufacture dependent. The pot life of resin can vary greatly depending on temperature and the type of resin used.

• Polyester resins are combined with a catalyst that leads to polymerisation. The pot life of polyester resins can be increased or decreased depending on the amount of catalyst used (usually between 1% and 4% of the resin weight). Always ensure to read the data sheet for guidance on specific resins. Therefore, it is easy to adapt the resin pot life by modifying the quantity of catalyst.

• Epoxy resins are combined with a hardener in order to achieve polymerisation. You should always respect the exact ratio indicated by the manufacturer between epoxy and hardener. If a longer working time is required, a slower catalyst may be necessary.

Ensure that you take into account the temperature and the time required when choosing your resin. If in doubt contact your supplier about working times. In both cases, it is imperative that the catalyst or hardener is mixed thoroughly to ensure an even cure.

What conditions will your product face?

It is important to assess the external environemental conditions that your finished piece will face during its lifetime.

• For basic repairs (car or bike bodies, camper vans, etc.), polyester resin is appropriate. If the piece is to be exposed to high chemical constraints, you may consider using Vinylester resins, which will be discussed in a further article.

• Epoxy resin is extremely strong and resistant; therefore, it is more suitable for cases where the product will face extreme conditions such as racing cars, planes or water sports. Epoxy resin is very resistant to osmosis, so it is the best choice for yachting applications, especially for parts under water.

For standard operations, polyester resin will fit your requirements. In the case of extreme conditions, epoxy will yield superior results.

In a nutshell...

Epoxy resin has very good adhesive properties, has good chemical resistance and offers some comfort while you use it (less fumes). However, polyester resin, often more than 3 times cheaper, fits most day to day general DIY jobs and using epoxy may be overkill.