Main Systems To Choose
Safety Items, Non Negotiable
Steps To Mixing It Right
Laminating Since
Main Types of Resin: Polyester vs Epoxy
Most beginner projects use one of two systems. Polyester is cheaper, cures quickly and suits general laminating and repairs. Epoxy costs more but bonds far better, shrinks less, and is the choice for structural work and for anything bonding to an already cured surface.
Polyester is where most people start. It wets out matting readily, cures in a couple of hours in normal conditions, and keeps the cost sensible when a job needs volume. It is catalysed with MEKP, and the amount of catalyst controls how long you have to work before it goes off.
Epoxy behaves differently. It uses a matched hardener rather than a catalyst, and the ratio is fixed by the manufacturer rather than adjustable by you. In exchange you get much stronger adhesion, particularly to timber, metal and cured laminate, and considerably less shrinkage as it sets.
There is a third system worth knowing about. Vinyl ester sits between the two, with chemical and corrosion resistance well beyond polyester, and it is what gets specified for tanks, marine work and anything living in salt or caustic service.
Safety Precautions You Must Take
Fibreglass resin is a reactive chemical, and the safety gear is not optional. Buy it at the same time as the materials, not after you have started.
Protection
Nitrile gloves, safety glasses and a respirator rated for organic vapour. Skin contact and vapour are both worth avoiding, every time.
Ventilation
Work outdoors or with genuine airflow. A closed garage with the roller door down is not ventilation.
Fire Risk
Curing resin generates heat. A bin of soaked rags can self ignite, so keep used rags in water and dispose of them properly.
Read the safety data sheet for the specific product before you open it. Storage conditions, shelf life, mixing ratios and disposal routes all vary between products, and the sheet is the only reliable source for the one in front of you.
Step-by-Step Guide to Mixing Resin
Mixing is where most first attempts go wrong, and it is entirely avoidable. Work through it in order and do not skip the test batch.
Mixing, in order
- Check conditions: temperature and humidity both affect cure. Below roughly 15 degrees most systems slow badly, and in high heat you lose working time fast.
- Prepare everything first: matting cut, surface sanded, tools laid out. Once it is mixed the clock is running and you cannot pause.
- Measure, do not estimate: use the ratio on the data sheet. For epoxy this is fixed. For polyester the catalyst level flexes with temperature, within the stated range.
- Mix thoroughly: stir for the full time stated, scraping the sides and the bottom of the cup. Unmixed material at the edge will never cure.
- Test a small batch: mix a little first and watch how it goes off. That tells you your real working time in today’s conditions.
- Work in small batches: mix only what you can lay down before it gels. A large cup generates heat and accelerates itself.
Almost every problem with fibreglass resin traces back to one of two things: a ratio that was estimated instead of measured, or a surface that was not properly prepared. Get those right and the rest tends to look after itself.
How to Apply Resin With Fibreglass Matting
The technique is less about skill than about patience and sequence.
Start by wetting the surface with a thin coat, then lay the matting into it rather than dropping dry cloth on and pouring over the top. Work the resin through with a brush using a stippling motion rather than a brushing one, which drags the fibres out of position.
Then use a laminating roller to drive out the air. Trapped bubbles are the difference between a laminate that lasts and one that delaminates, and they are far easier to remove now than to fix later. Roll from the centre outward and keep going until the mat looks translucent rather than white.
Build thickness up in layers rather than trying to achieve it in one hit. Each layer should go on while the previous one is still slightly tacky, which gives a chemical bond between them. If a layer has fully cured, sand it before the next goes on.
Not sure which system your job needs? We laminate with these materials every day at our Wingfield workshop. Tell us what you are making and what it has to survive, and we will point you at the right product and where to get it, or quote the work. Have a look at our fibreglass resin guide or the advanced composite components we build.
Frequently Asked Questions
Common questions, answered by our Adelaide team.
How much catalyst should I add?
Follow the ratio on the product data sheet rather than a rule of thumb, because it varies between products and with temperature. As a principle, warmer conditions need less and cooler conditions need more, always within the range the manufacturer states. Never go outside that range to speed a job up. Too much generates excessive heat and can crack the laminate, and too little leaves the surface tacky or uncured indefinitely.
How long does fibreglass resin take to cure?
Polyester systems typically gel within tens of minutes and reach a workable hardness in a few hours, while a full chemical cure takes considerably longer. Epoxy generally takes longer to gel but behaves more predictably once it starts. Both are heavily affected by ambient temperature, so a job that behaves one way in summer will behave quite differently in a cold shed in July. Test a small batch first, every time.
Can I apply resin over an existing repair?
Yes, provided the surface is properly prepared. Sand back to remove gloss and any wax or release agent, clean off the dust, and make sure the surface is dry before you start. Epoxy bonds to cured surfaces considerably better than polyester does, which is why it is the usual choice for repairs to something already in service rather than for a fresh layup in a mould.
