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Windshield Repair Kits, Explained

After reading, you'll understand what kind of damage a kit can legitimately fix, where on the glass repair stops being allowed, why resin thickness matters, and what a finished repair should and should not look like.

Updated August 23, 2026 · Companion to our Automotive review

In this guide

Most people buy a windshield repair kit in a hurry, on the assumption that a chip is a chip. Then they discover three things at once: that the break they have is a shape with a name and a size limit, that where it sits on the glass matters more than how big it is, and that even a textbook repair leaves something visible. None of that is hidden. It is written down in an ANSI standard that the repair industry wrote for itself, and ten minutes with a tape measure will tell you more than any amount of packaging.

The 30-second version

  • Damage has a shape and a size limit, and the standard sets them by type: one inch for a bullseye, two inches of body for a combination break, three inches for a star break, 14 inches for a single crack.
  • Location beats size, because in the 12-inch-wide band of glass directly in front of the driver, running the full height of the wiper sweep, the rules tighten sharply and some damage should be replaced rather than repaired.
  • The tool exists to move air, pulling it out of the break under vacuum and replacing it with resin under pressure, which is why an injector that locks beats an applicator you hold.
  • Thin resin and thick resin do different jobs, with low viscosity resin wicking into tight cracks and legs while thick pit resin fills and polishes the surface flush.
  • A good repair is still visible, and the standard even has a name for the leftover mark, so judge the work by whether the damage stopped spreading.

The products this guide is about

Our verdict on each, with the full reasoning on the companion review.

What actually matters

  • Repairable dimensions - The standard gives a maximum size for each damage type rather than one blanket number. Knowing whether you have a bullseye, a star break or a combination break tells you which limit applies.
  • Driver's Primary Viewing Area - This is the band of glass 12 inches wide, centred on the driver, spanning the wiper sweep from top to bottom. Inside it, repair is off the table once damage passes one inch, once the finished pit would exceed 3/16 inch, or if the repair would land within four inches of another repair.
  • Vacuum and pressure - Repair is defined as removing air from the break by vacuum or displacement and then filling it with resin. A kit that cannot hold a vacuum is asking resin to push air out of a dead end, which it does badly.
  • Resin viscosity - Thin resin travels into hairline legs and cracks, thick resin stays where you put it and builds a polishable surface. Professional kits carry both because one liquid cannot do both jobs well.
  • Cure method - Consumer resins cure in sunlight, which ties you to weather and time of day. A UV lamp removes that constraint and lets you cure on your schedule.
  • Glass temperature - Resin flow and cure both change with temperature, which is why instructions tell you to park in shade and get the glass into a stated window before starting.
  • Value-added features and ADAS - Rain sensors, heads-up displays and forward-facing cameras occupy specific areas of the windshield. The standard says not to repair where those could be affected, and to follow the vehicle maker's recommendations.

The specs, in plain English

BullseyeA round break marked by a separated cone in the outer layer of glass, showing as a dark circle with an impact point in the middle. It is the friendliest damage to repair. The standard calls it repairable up to one inch, or 25 mm, in diameter.
Star breakDamage with a series of cracks, called legs, radiating out from the impact point. Resin has to travel down every leg, which is why vacuum matters more here than on a bullseye. The standard puts the repairable limit at three inches, or 75 mm, across the break.
Combination breakDamage with more than one characteristic at once, such as a star sitting inside a bullseye, or cracks running out of the break. It is measured by the diameter of the body, excluding the legs, and is repairable up to two inches, or 50 mm.
LegsThe subsurface cracks that spread out from a break. They are the part of the damage that keeps growing when the glass flexes or the temperature swings. Filling them completely is the difference between a repair that holds and one that runs across the windshield next winter.
Pit and finished pitThe pit is the impact point where a small piece of glass is actually missing. The finished pit is what remains after you fill and polish it flush. The standard says a finished pit should not be larger than 3/8 inch, and no larger than 3/16 inch inside the driver's viewing area.
Edge crack, floater crack and stress crackAn edge crack reaches an edge of the glass, a floater crack does not, and a stress crack runs from an edge without any impact point, usually from flexing or bad mounting. The distinction matters because the standard says not to repair stress cracks at all, or edge cracks that intersect more than one edge.
PVB or plastic interlayerA windshield is two layers of glass bonded to a plastic interlayer, usually polyvinyl butyral. That layer is what holds the glass together when it breaks and keeps occupants inside the vehicle. If the damage reaches or discolours it, or goes through both layers of glass, repair is off the table.
Refractive indexHow much a material bends light passing through it. Repair resin is formulated to approximate the refractive index, colour and clarity of the glass, which is what makes a filled break far less noticeable. It never matches perfectly, which is why you can usually still find the repair if you look.
Viscosity in cpsCentipoise is the unit for how thick a liquid is. A crack resin around 20 cps flows like water and wicks into tight separations, while a pit resin around 3000 cps is closer to syrup and stays on the surface to be cured and polished. Consumer kits usually ship one resin somewhere in between.

Green flags vs red flags

Green flags

  • The maker publishes a maximum damage size in inches or millimetres rather than vague language about chips.
  • The kit uses a pedestal that seals to the glass plus an injector that locks into separate vacuum and pressure positions.
  • Instructions state a glass temperature window and tell you to work in shade.
  • The packaging or page admits the repair may leave a visible blemish and may not fully fill the pit.
  • There is a curing strip or UV lamp and a stated cure time, not just an instruction to wait.

Red flags

  • Marketing that promises an invisible or like-new result.
  • One small bottle of general purpose resin sold as a fix for both chips and long cracks.
  • No mention anywhere of where on the windshield you should not attempt a repair.
  • An applicator you have to hold in place by hand for the whole cycle instead of a locking pedestal.
  • Claims to repair damage that goes through both layers of glass, or damage on the inside surface.

Who's who: the brands

  • Versachem - Automotive chemical brand whose windshield kit publishes an unusually specific vacuum and pressure sequence for a shelf product.
  • Permatex - Long-running automotive chemicals maker whose windshield kit is aimed at bullseye damage and is candid about the cosmetic result.
  • Rain-X - Best known for glass treatments, with the most widely stocked consumer repair kit and the broadest damage claims.
  • J-B Weld - Adhesive and epoxy specialist whose Windshield Saver kit uses a pressure-lock pedestal and a sun-activated acrylic resin.
  • Delta Kits - Professional windshield repair equipment maker that sells bridges, injectors and separate bond and pit resins by viscosity.

How to read a listing without getting fooled

Read a windshield repair kit listing in this order. First find the damage size and type the maker actually claims, in inches or millimetres, and compare it against the break in front of you rather than against the photo on the box. Second, look at the applicator: a pedestal that adheres to the glass with an injector that locks in two positions is doing more work than a device you press and hold. Third, find the resin quantity and any viscosity information, and treat a single small bottle as a chip solution regardless of what the crack claim says. Fourth, look for working conditions, meaning a glass temperature range, an instruction to work in shade and a stated cure time in sun or under a lamp. Anything that skips all four and leads with a before-and-after photo is selling the outcome rather than the process.

How much should you spend?

Consumer windshield repair kits sit firmly in the budget tier, and the spread between the cheapest and the most complete shelf kit is small enough that it should not drive your decision. Pay for the injector design and the published instructions rather than the brand on the clamshell. The genuine step up is to a professional bridge system in the premium tier, which is a different order of magnitude and buys you a screw injector, separate bond and pit resins, a UV lamp, a drill and enough consumables for dozens of repairs. That only pays off if you expect to do many repairs, and it does not pay off at all if you are going to attempt damage the standard says should be replaced. If the damage is outside the repairable limits or sits in the driver's viewing area, the money is better spent at a shop.

Questions, answered

What is the resin actually doing inside the break?

It is filling a void that is currently full of air. The tool removes that air by vacuum or by displacement, and the resin flows in to take its place, then cures into a solid that seals the break and bonds the separated glass. Because the resin is formulated to approximate the refractive index, colour and clarity of the glass, the filled area scatters much less light than an air-filled break. That is why a repair looks better and, more importantly, why the damage stops spreading.

Why do the instructions care so much about shade and glass temperature?

Temperature changes how the resin flows and how fast it cures. Hot glass in direct sun can start curing the resin before it has reached the ends of the legs, leaving voids that will fail later. Cold glass makes the resin sluggish and slow to travel. That is why kits tell you to move the car into shade and wait for the windshield to reach a stated range, commonly around 50 to 75 degrees F, before you start, and only then move it into the sun to cure.

Does it matter how long the chip has been there?

Yes, quite a lot. Dirt, road film and moisture work their way into the break over days and weeks, and the standard says damage contaminated with visible impurities that cannot be removed by cleaning should be replaced rather than repaired. Contamination also stops the resin bonding cleanly, so an old chip can look repaired and still spread. If a chip is fresh, cover it with clear tape to keep water and grit out until you can deal with it.

Can a finished repair fail later?

It can, and the standard tests for exactly that. Resins seeking recognition under ROLAGS have to survive a thermal cycling test in which repaired samples are equilibrated at 0 degrees F in a freezer and then heated from below until the repair side reaches 150 degrees F, repeated up to three times. A conforming repair should come through unchanged, with no separation of the star portion from the glass or the interlayer and no voids appearing in the bullseye. That cycle is a stand-in for a cold morning and a hot defroster, which is when marginal repairs let go.

Ready to choose? See our top pick, the alternatives, and the one we'd skip See the Windshield Repair Kits verdicts

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