How Long Does Resin Cure in Windshield Repair?

Ask five technicians how long resin takes to cure in a windshield chip repair, and you might hear five different answers. They are all telling the truth from their angle. Cure time depends on resin chemistry, the power and wavelength of the curing light, ambient temperature, the depth and type of break, and even how clean the damage was before injection. I have spent mornings watching resin behave perfectly under a bright summer sun, then spent afternoons coaxing the same product to finish under a weak shop lamp with a cold wind pushing through the bay. The timeline is elastic, but not mysterious, once you break it down.

What “cure” actually means in glass repair

Curing is the chemical crosslinking that turns liquid resin into a solid polymer within the chip or crack. You are not just drying something; you are initiating a reaction that changes the material. Most windshield repair resins are UV‑curable acrylates engineered to match the refractive index of laminated glass. They stay fluid until exposed to UV light in the right band, mostly UVA, typically 365 to 405 nanometers. Under UV, a photoinitiator kicks off polymerization, and the resin hardens in place, bonding to the micro‑etched glass and displacing air and moisture.

Full cure is different from cosmetic cure. Cosmetic cure is when the repair looks clear and stable to the eye. Full cure is when the polymer has reached the degree of conversion needed to resist thermal movement, wiper strikes, and vibration. That second one is the benchmark if you want a repair that holds up through winter.

The quick answer, with realistic ranges

If you want a working number: under a good professional UV LED lamp at 365 to 395 nm, most pit fillers cure at the surface in 20 to 60 seconds, and injection resins inside the break cure in 2 to 5 minutes. Under direct midday sun, the surface can skin in under a minute, but the body of the repair often needs 5 to 10 minutes for a confident set, especially in cooler air or on thicker glass. With older fluorescent UV sticks or weak LEDs, double those times.

Now for the nuance that makes those numbers honest in the real world.

Variables that actually change cure time

Temperature sets the pace. Resin thickens when cold, slowing both flow and cure. On a 40 degree morning, I budget roughly twice the cure time compared to a 75 degree afternoon. In a hot parking lot, the inverse is true, provided you control the windshield temperature so you do not trap bubbles or open a crack with thermal stress.

UV intensity matters just as much as temperature. A 5,000 to 10,000 microwatt per square centimeter LED at the correct wavelength cures resin rapidly and consistently. If your lamp is older or the lens is scratched or cloudy, effective intensity drops and the edges of the break do not see the same dose as the center. Sunlight is powerful but fickle. A thin cloud can cut UVA significantly while still feeling bright to your eyes. Angle of the sun, tint band at the top of the windshield, and the shadow from the vehicle’s roof all change the dose.

Glass thickness and geometry play a quiet role. Large SUVs and trucks typically have thicker laminated glass. The deeper the resin sits in a star break or combination break, the more attenuation you get before UV reaches it. Tight legs in a star chip can hide shadows. That is the spot that makes you doubt your cure if you rush.

Resin type is not one thing. Low‑viscosity injection resins that wick into tight legs are formulated to cure efficiently with moderate UV. High‑viscosity pit fillers, used to level the surface, often include heavier loadings of fillers to sand cleanly and resist abrasion. Those fillers can extend the surface cure time by a minute or two, especially under weak lamps.

Contamination and moisture are cure killers. If the chip has held rainwater, washer fluid, or oil, the resin may not bond evenly, and unreacted pockets can linger longer than you think. You can flood UV at it and still not get a trustworthy cure if the substrate was wet.

The anatomy of a proper cure, step by step

Think of the cure in three arcs: air control, structural cure, and surface finish. If you hit all three without hurry, your repair will be clear and durable.

• Air control comes first. Vacuum and pressure cycles draw air out of the break and replace it with resin. If any air remains, the UV has to pass through unevenly. Allow your injector cycles to finish fully, then let the resin sit under slight pressure to equalize, especially in multi‑leg cracks.

• Structural cure is the heart. After removing the injector, apply your UV light with the correct distance and angle. I prefer to anchor the lamp directly over the break so it cannot drift. Hold it for a minimum of 2 minutes under a strong LED for most star breaks, extending to 3 to 4 minutes for combos. In cold weather, 4 to 6 minutes is cheap insurance.

• Surface finish is the last step. After scraping the surface flush and applying pit filler under a curing tab, use short, focused UV exposures. Thirty seconds under a strong lamp is plenty for many pit fillers, 60 to 90 seconds if your lamp is weaker or the tab is thick. Peel, scrape again, and give it a brief post‑cure if you see any drag on the blade.

That sequence may look simple, but the discipline of holding enough UV time, especially after a good cosmetic result appears early, is what separates short‑term repairs from the ones that ride through summer heat.

LED lamp power, wavelength, and placement

A UV LED labeled as 365 nm or 395 nm covers the useful band for most resins. The exact wavelength affects speed and depth. Many modern resins respond quickly at 395 nm because the photoinitiators are tuned for it, while 365 nm drives deeper penetration. If you can choose, a lamp that offers both or sits near 385 to 395 nm is a good all‑around tool.

The number on the box is not the whole story. Real‑world intensity at the glass depends on:

• Distance between the lamp and the surface.
• Angle of incidence relative to the break.
• Cleanliness of the lamp lens and shield.

An inch away at 90 degrees beats three inches at a slant. I have tested lamps with a UV meter and seen a 30 to 50 percent drop in intensity at two inches compared with one. That translates directly to longer cure time. Wipe your lamp lens. Fine resin mist builds up over months, and you will not notice until your cure creeps.

If you are using sunlight as your source, give yourself margin. Aim for high sun, adjust the vehicle so the break sits in full light, and hold 5 to 10 minutes for structural cure, even if the pit filler skins fast. A thin cloud can stretch that to 10 to 15. When in doubt, double the time rather than crossing your fingers.

How chip type changes the clock

Not all breaks are created equal. A bull’s‑eye, cleanly circular and shallow, is the easiest to fill and cure. A minute or two of UV under a good LED usually locks it down.

Star breaks with long, tight legs demand patience. Resin can take longer to reach the tips, and UV takes longer to reach the same spots. I like to give star breaks an extra minute beyond my baseline, then flex the glass gently at the legs with a probe before the final cure to ensure resin has reached the end. If you see movement at the tip, hold another minute.

Combination breaks stack challenges. You may have a partial bull’s‑eye, some star legs, and surface pitting. Fill those in the usual order, but do not be shy about 3 to 4 minutes of structural cure before you touch the surface. The cosmetic finish is faster when the core is truly set.

Long cracks are their own category. Many technicians use a crack resin with lower viscosity and cure in segments, leapfrogging the lamp along the crack. Each segment may cure in 30 to 90 seconds under a strong LED, but the total job accumulates. In cold conditions, I will often run a slow final pass with the lamp along the entire crack to even out the cure.

The weather factor you feel in your bones

Anyone who repairs glass outdoors learns to read the weather like a sailor. Cold is the obvious problem, but wind can be worse. A 55 degree day with a stiff breeze will pull heat out of the glass, slowing polymerization, and the vibration can coax a small crack to run if the glass is already under stress. Shield the area. A simple cardboard baffle or a shop apron clipped near the repair keeps wind off the lamp and the glass, boosting both resin flow and cure reliability.

Heat is friendlier to cure, but it brings risk. If the glass is hot to the touch, the laminate is expanding. Flooding UV instantly can trap small bubbles that would have off‑gassed if you allowed the glass to cool a few minutes or worked in the shade. Also, hot glass can soften the pit filler just enough that it smears under the blade. Let the surface temperature drop below roughly 90 to 100 degrees before the cosmetic steps. An infrared thermometer is inexpensive and removes guesswork.

Humidity itself does not stop UV cure, but it often arrives with condensation on cool mornings. If the chip is wet, no amount of UV time will fix adhesion issues. Warm the area gently with a moisture evaporator or a heat gun on low. Two to five minutes of controlled warmth, then a short cool‑down, makes your cure time meaningful again.

How to know when the resin is actually cured

There are simple cues you can trust without lab instruments. The blade test is my first move. After your structural cure, scrape the surface lightly with a fresh razor. If it shaves clean and the material throws crisp, glassy shavings, you likely have a good cure. If it smears or drags like soft plastic, the surface filler needs more light, or the underlying resin is still green.

At the pit, a fingernail tap gives feedback. A sharp, high click suggests rigidity. A dull thud suggests a gummy layer under the surface. With experience you will hear the difference. You can also use a probe at the edge of the repair to check for flex. Solid resin does not indent easily.

If you keep a small UV meter, you can check your lamp’s output occasionally. It is not a daily ritual, but it prevents surprises. When a once‑reliable lamp starts requiring an extra minute on every job, you will see it in the number before customers start seeing it in failed repairs.

Why the pit filler seems to take forever sometimes

Most complaints about cure times come from the surface step, not the internal structural cure. Pit filler is a different animal. It includes silica or other fillers that give it body, sandability, and abrasion resistance. Those particles scatter light and can slow cure a bit. The curing tab you place over the filler also matters. Thick, cloudy tabs block more UV. If you flip between brands of tabs and notice cure time swing by 30 to 60 seconds, that is normal.

You can help the process. Use a clear, clean curing tab. Smooth the filler thinly and evenly so you do not create a dome that takes longer to penetrate. Aim for the lamp to sit nearly flush to the tab, within a half inch if possible, for a reliable 30 to 60 second set under a strong LED. If you are working under winter sun, give it 2 to 3 minutes and do not rush the scrape.

Can you drive right away after a repair?

Yes, in most cases, once the resin is fully cured and the pit is scraped flush, the vehicle can be driven immediately. The repair is not just dry, it is a solid polymer bonded within the glass. That said, strong caution applies to automatic car washes and power washing the area for the first few hours. The pit filler keeps the surface sealed from abrasion, but aggressive brushes right away can nick a fresh surface, especially if the cure was marginal. I tell customers to wait until later in the day for a wash if we finished the job in the morning, or until the next day if we repaired late afternoon in cold weather.

The connection to windshield replacement

Every proper repair sidesteps a potential windshield replacement. The resin does not make a break disappear entirely, but by curing fully and bonding the layers, it restores structural integrity to the area. A half‑cured repair is worse than no repair. It can leave a cosmetic haze and a weak spot that spreads with the first hard frost or pothole hit. If a break is in the driver’s direct line of sight, very large, contaminated, or already running long beyond a manageable length, a repair may not be responsible. In those cases, windshield replacement is the safer path. The cure timeline is not the limiting factor then, clarity and safety are.

From a cost angle, a chip repair that takes 20 to 30 minutes, including a careful cure, is far cheaper than the parts and labor for a replacement. It also preserves the factory seal, which matters for cabin noise and leak prevention. But only if the cure is complete.

The risk of overcuring and heat buildup

Technicians worry about undercuring, but you can also overcook a repair. Long, intense UV exposure raises localized heat. On a hot day, with a big LED pressed tight to the glass for 10 minutes straight, you can stress the laminate. If the crack is pre‑tensioned from heat, the extra thermal bump can nudge it. The solution is simple: cure in sensible stages. Two to four minutes for structural cure, assess, then another minute if needed, rather than parking a 10‑watt lamp on one spot for ten minutes.

On the surface, overcuring pit filler is less risky but can make scraping harder. A rock‑hard ridge will fight the blade and tempt you to angle the razor too steeply, risking a scratch. If the pit filler feels like granite, a few light passes with a fresh blade will still shave it clean. Resist the urge to bear down.

Field stories that sharpen your timing

Two different mornings make the timing lesson stick for me. A March job, cloudy and 38 degrees, on a dark SUV that had sat overnight. The chip looked simple, a small combo. The first cosmetic pass looked great after two minutes under a shop UV stick. I almost called it done. A gentle probe at a star leg showed movement at the tip. I gave it three more minutes under an LED I grabbed from the truck, then the same probe test felt solid. That repair came back for a revisit a year later after a new chip, and the original spot still looked tight.

Another day, July heat, full sun, a shallow bull’s‑eye on a compact. The resin filled easily and skinned fast. Thirty seconds of pit filler cure, scrape, and I thought I was free. The blade dragged. I had shaded the area with my body by accident, and the tab had a light haze. Ninety more seconds with the lamp pressed close, and the blade shaved cleanly. Small details, short delays, big difference in finish.

Practical timing benchmarks you can trust

Use ranges, and let conditions push you up or down within them.

• Structural cure under a strong LED: 2 to 3 minutes for bull’s‑eyes and small stars; 3 to 4 minutes for combination breaks or cold weather; 5 to 6 minutes if your lamp is weaker or the glass is thick and cool.

• Surface pit filler under a clear tab: 30 to 60 seconds under a high‑output LED close to the glass; 90 seconds to 3 minutes in weak light or under cloud; up to 5 minutes in cold shade if sunlight is the only source.

If you are curing a long crack in segments, treat each 2 to 3 inch section with 30 to 60 seconds, then run a slow, overlapping pass for one minute along the entire length at the end. Adjust upward if the air is cold or wind is strong.

Mistakes that stretch cure time and how to avoid them

The most common time sink I see is rushing the initial moisture removal. If a chip has held water, spend those extra two to five minutes with gentle heat and a brief cool‑down. Your cure time later will be shorter and more certain. Second is neglecting lamp maintenance. Keep a dedicated wipe for the lamp lens; a clear lens restores intensity. Third is fighting the weather without barriers. A simple windbreak and a shade panel can turn a 10 minute cure into a reliable 4.

Finally, trust your instruments and your senses. An inexpensive IR thermometer and a periodic UV intensity check keep your timing grounded. Your ears, eyes, and fingertips close the loop.

When cure time tells you to walk away

If you find yourself doubling and tripling the cure time and still seeing smear, drag, or flex, you likely have a contaminated or delaminated area, or a break that has already absorbed oils or washer fluid long enough to resist bonding. That is the sign to stop, explain the situation to the customer, and recommend windshield replacement instead of forcing a questionable repair. A fast fix that fails is not a service. A clear recommendation is.

Final take: the clock is yours to control

Resin cure in windshield repair is not a mystery timer hidden in the bottle. It is a function of light, temperature, geometry, and patience. With a strong UV LED at the right distance, a clean, dry break, and a little weather sense, most repairs reach a solid structural cure in 2 to 5 minutes, with the surface finishing in under 90 seconds. Stretch those numbers in cold, wind, or weak light, and tighten them when conditions favor you, but never let a clear look fool you into switching off the lamp early. The road will test whatever you leave inside the glass.

Do the simple things right every time: warm and dry the break when needed, center a clean lamp close to the surface, hold the light long enough for depth, and finish the pit with a clear tab and a steady blade. You will send more cars back on the road with a repair that outlasts the seasons, and fewer toward windshield replacement that could have been avoided.