Polyester is the workhorse chemistry of stone repair. It fills chips, rebuilds broken corners, bonds rodding into a sink cutout, and takes color well enough to disappear into a busy granite. It is also the material fabricators most often get wrong, because unlike a two-part epoxy with a fixed volumetric ratio, polyester is catalyzed rather than mixed in a stoichiometric proportion. You are not combining two halves of a reaction; you are adding a small quantity of initiator that triggers the resin to cure itself. That distinction changes everything about how the dose should be handled.
Get the catalyst dose right and polyester is fast, sandable, colorfast and strong. Get it wrong in either direction and you get failures that are frustrating precisely because they look like product problems. Too little catalyst leaves a repair that stays tacky for hours, never reaches full hardness, and pulls out under the first polishing pass. Too much produces a batch that gels in the cup before you can place it, cures brittle, and can discolor or crack over time. This guide covers where the working window sits, how temperature moves it, and how to run repeatable batches in a real shop.
How Catalyzed Polyester Differs From a Mixed-Ratio Adhesive
A two-component epoxy uses a resin and a hardener that react with each other in defined proportions. Change the ratio and you leave unreacted material in the cured mass, which is why epoxy ratios are unforgiving. Polyester works differently. The resin contains everything needed to cure; what it lacks is a trigger. The catalyst, typically methyl ethyl ketone peroxide, decomposes and generates free radicals that start the cross-linking chain reaction. Once started, the reaction propagates through the resin largely on its own.
The practical consequence is that catalyst percentage is a rate control, not a composition control. Adding more catalyst does not make the repair stronger; it makes the cure start sooner and proceed faster. Adding less does not make it weaker in principle, but below a threshold the reaction fails to propagate fully and you are left with a soft, under-cured mass. Understanding the dose as a speed dial rather than a strength dial prevents most of the reasoning errors fabricators make with this material.
The generally published working range for polyester resin is approximately one to two percent catalyst by weight of resin. That range exists because it reliably initiates a complete cure while leaving usable working time. Published guidance also warns against exceeding roughly three percent maximum catalyst, above which the product may fail to cure properly, or the cured material may shrink, deform, or end up hard and brittle. Treat three percent as a hard ceiling, not as an option for a fast job.
Working the Dose in the Shop
Temperature Is the Main Variable
Cure rate is strongly temperature dependent, and the same dose behaves very differently across a shop's seasonal range. Published guidance for polyester puts a two percent catalyst dose at roughly a ten to fifteen minute gel time at around seventy-seven degrees Fahrenheit. That figure is a useful anchor because it gives you a known point to reason from rather than guessing from scratch.
From that anchor, the compensation rule is straightforward: add less catalyst in warmer conditions to regain working time and slow the cure, and add more catalyst in cooler conditions to shorten working time and speed the cure. What fabricators frequently miss is that the relevant temperature is the temperature of the resin and the stone, not the air. A jug of polyester that has been sitting in a cold van behaves like cold resin for a long time after it comes inside, and a slab pulled from an unheated yard pulls heat out of a repair fast enough to stall the cure at the bond line.
Measuring the Dose Reliably
Eyeballing catalyst drops into an unmeasured blob of resin is the most common source of inconsistency, and it is entirely avoidable. Work by weight on a small digital scale: tare the cup, weigh the resin, and calculate the catalyst quantity from that mass. A cheap scale reading to a tenth of a gram turns a guessing game into a repeatable process and lets you record what actually worked on a given day.
If you must dose by drops, calibrate your dispenser once and write the result on the bottle. Drop volume varies significantly with the dispenser tip, the fluid temperature and how hard the bottle is squeezed, so a count that works with one bottle is not transferable to another. Recalibrate whenever you open a new container or change dispensers.
Mixing Technique
Mix thoroughly but without whipping air into the batch. Entrained bubbles show up as pinholes when the repair is ground back and polished, and they are impossible to fix without redoing the fill. Fold and scrape rather than stir vigorously, and make certain you reach the sides and the bottom of the cup, because unmixed resin against the cup wall is a frequent cause of a soft edge on an otherwise good repair.
Mix only what you can place. Because the reaction is exothermic and heat accelerates it further, a large mass in a cup will gel dramatically faster than the same chemistry spread thin on a stone surface. A batch that would give you twelve minutes as a thin fill may give you four minutes as a deep pool in a mixing cup. Small, frequent batches beat one large batch almost every time.
| Condition | Dose Direction | What to Watch For |
|---|---|---|
| Warm shop, warm slab | Move toward the lower end of the range | Batch gelling in the cup before placement |
| Cold shop or cold slab | Move toward the upper end of the range | Soft, tacky surface that never fully hardens |
| Deep fill or large mass | Reduce dose; consider multiple lifts | Exotherm accelerating the cure and cracking the fill |
| Thin skim over a large area | Slightly higher dose within range | Surface staying tacky from air inhibition |
| Vertical or overhead repair | Thicken rather than over-catalyze | Sag; do not fix sag with extra catalyst |
| Color-matched repair | Keep dose consistent batch to batch | Shade drift between batches at different cure rates |
Pro Tip
Run a scrap test before committing to a visible repair, especially on the first job of a cold morning or the first job after a heat wave. Mix a small batch at your intended dose, place it on an offcut of the same stone, and time the gel. Two minutes of testing tells you exactly how the material will behave on the piece that matters, and it costs a thimble of resin instead of a remake.
Color Matching, Depth and Repair Sequence
Coloring paste is added to the resin before catalyst, not after. Pigment loading changes the mass of the batch and therefore the catalyst quantity if you are dosing by weight, so weigh the tinted resin rather than the untinted resin when you calculate. This is a small detail that quietly shifts your effective percentage when heavy pigment loads are used for very dark stones.
Match color wet and check it cured. Polyester typically shifts slightly as it cures and again as it is polished, so a match that looks perfect in the cup will often read differently on the finished surface. Keep small cured chips of your standard color blends on a board as a reference library; matching against a cured sample is far more reliable than matching against liquid resin.
For deep repairs, build in lifts rather than filling in one pour. A deep mass of polyester generates enough internal heat to accelerate its own cure unevenly, which produces shrinkage stress and can crack the fill or the surrounding stone. Filling in two or three shallower stages, allowing each to cure, gives a dimensionally stable repair with far less risk. It also lets you keep the catalyst dose in the comfortable middle of the range instead of dropping it to fight exotherm.
Surface preparation matters as much as chemistry. The bond depends on clean, dry, sound stone. Slurry residue, sealer, dust or moisture in a chip will all compromise adhesion regardless of how well the resin is catalyzed. Clean with an appropriate solvent, let it flash off completely, and place the repair while the surface is genuinely dry rather than merely looking dry.
Air inhibition is worth understanding because it explains a common complaint. The top surface of a polyester repair exposed to air can remain slightly tacky even when the body has cured hard, since oxygen interferes with the surface reaction. Covering the repair with tape or film while it cures excludes air and gives a fully hard surface, which also reduces the amount of material you have to grind away.
Storage, Safety and Batch Discipline
Catalyst and resin both have finite shelf lives and both degrade faster when stored warm. Keep them in a cool, dark, well-ventilated location away from heat sources and out of direct sun. Old catalyst loses potency, which produces slow or incomplete cures that fabricators typically misdiagnose as a bad batch of resin. Date containers when they arrive and rotate stock so the oldest is used first.
Store catalyst separately from resin and away from any accelerator or promoter. Organic peroxide catalysts and accelerators must never be combined directly; they should only meet after each has been dispersed into the resin. This is a genuine safety issue rather than a process nicety, and it is why catalyst goes into the resin as the last step in the sequence.
Personal protective equipment is not optional with this chemistry. Catalyst is a strong oxidizer and is aggressive to skin and especially to eyes, so eye protection and chemical-resistant gloves belong on before the bottle is opened. Provide adequate ventilation at the repair station, since polyester releases styrene vapor during cure and repair benches are often tucked into poorly ventilated corners of a shop. Read and keep the safety data sheets for the specific products you stock.
Dispose of waste thoughtfully. Rags and mixing cups containing catalyzed resin continue to react and generate heat after you set them down, and a bin full of them can get genuinely hot. Let waste cure fully in a metal container away from combustible material rather than dropping it into a bin of paper and sawdust, and follow local requirements for disposal of the cured material and empty containers.
Finally, build a simple log at the repair station. Record the date, the ambient and slab temperature, the resin lot, the dose you used and how the batch behaved. Within a few weeks you will have a table that tells any operator what to dose on a given kind of day, which converts an experience-dependent skill into a shop procedure. That log is also the fastest way to diagnose a genuine product problem, because you will be able to show that the same dose behaved differently with a specific lot.
Humidity deserves a mention as well. Very damp conditions can leave a film of moisture on cool stone that is invisible but sufficient to weaken the bond, and they slow the surface cure of an exposed fill. Warming the stone slightly before placing a repair, or simply moving the piece into a conditioned part of the shop for an hour, removes a variable that is otherwise easy to blame on the resin.
Consistency compounds. A shop that doses by weight, tests on scrap when conditions change, works in lifts on deep fills and logs its results will produce repairs that hold up through fabrication, installation and years of service. A shop that squeezes drops by feel will keep producing occasional failures that are expensive to chase down after installation.
Our stone adhesives range covers polyester, epoxy and acrylic systems for different repair and bonding situations, and the color matching products let you build a repeatable palette for the stones you handle most. Matching chemistry to the specific repair is usually a bigger factor in the outcome than technique alone.
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