What a Stone Chip Actually Does to Your Paint
An owner stood next to his bonnet last week with his thumb on a mark the size of a grain of rice. Eight months old, bought new, driven on the Eastern most days. He wanted to know whether it was worth worrying about or whether he was being precious. It is the most common question I get in the workshop and the honest answer is that it depends entirely on how deep it went, which is something you cannot tell from standing up.
So here is what is actually under your thumb.
Your paint is four layers, not one
What owners call "the paint" is a stack, and each layer does a different job.
At the bottom is bare metal or plastic. Over that goes an electrocoat and a primer, which is the layer that stops corrosion and gives everything above it something to hold onto. Then comes the basecoat, which is the colour, and it is thinner than people expect. On most modern cars the colour layer is around fifteen to twenty microns, roughly a fifth of the thickness of a human hair. On top of all of it sits the clear coat, the thickest single layer on the car, and the only one doing any physical defending.
Add the whole stack together and on a factory finish you are looking at roughly one hundred to a hundred and forty microns. About the thickness of a sheet of paper, standing between the weather and the steel.
What the stone does on the way in
A chip is not a scratch. A scratch drags. A chip is an impact, and the energy has to go somewhere.
At highway speed a bit of bluestone off the back of a truck arrives with enough force to shatter the clear coat rather than dent it. The clear coat is hard and brittle by design, which is what makes it good at resisting swirls and bad at absorbing a hit. It fractures, lifts, and takes a flake of itself away. If the energy stops there, you have a white or grey speck and nothing more serious than a cosmetic annoyance.
If it goes further you lose the colour layer too, and you are looking at primer. That still is not an emergency, but the countdown has started, because primer is porous.
If you can see something metallic and bright in the bottom of it, water is now touching steel. In a Melbourne winter that is a rust bloom under the surrounding paint within a season or two, spreading outward under a film of colour that still looks fine from two paces back.
The test I use is simple. Run a fingernail across it. If the nail catches and the bottom of the chip is the same colour as the car, it is shallow. If you can see a different colour down there, it is through.
Why Melbourne is harder on paint than most owners assume
Two things stack up here. The first is the surfacing. A lot of our arterials and freeway sections are finished with a coarse chip seal, and the loose aggregate sitting on top of a fresh seal for the first few weeks is exactly the size and hardness that does damage. Anyone who has followed a truck out past Ringwood after a resurface knows the sound.
The second is the driving. Most of the front end damage I see is not one dramatic event. It is thousands of kilometres of the same commute, the same stretch of the Monash or the Eastern, picking up a speck at a time until the bonnet leading edge and the bumper go from clean to peppered. Owners rarely notice it happening. They notice it when the car is being sold.
Touch up paint is not a repair
A touch up pen fills the hole and seals the metal from water, which is worth doing if you are down to primer or bare steel. It buys you time and it stops corrosion.
What it does not do is repair anything. The dab sits proud of the surrounding clear coat, holds a different sheen, and catches the eye from a metre away once the light is right. On a car being kept, that is a reasonable trade. On a car going to market, a bonnet full of dabs reads worse to a buyer than a bonnet full of chips, because dabs look like a problem somebody tried to hide. Properly correcting a chip means filling it in stages and levelling it back to the surrounding clear coat, which takes far longer than most people imagine.
Where film sits in all of this
Paint protection film is the only thing that stops a chip happening in the first place. A ceramic coating will not. Coating makes a car dramatically easier to keep clean and keeps swirls down, and it is good at what it does, but it is measured in microns and it will not absorb an impact. Anyone telling you otherwise is selling you something.
That said, film is not for everyone, and I ask every owner weighing it up the same question. If stone chips were never a concern on your last car, they will not be on this one, and film will add nothing for you. Half of what it buys is emotional value, and there is nothing wrong with that, but it should be your decision rather than one made for you at a handover desk.
If you do want it, timing matters more than the film brand. Film goes over what is underneath it. A brand new panel stays brand new. A peppered panel stays peppered, preserved under ten year film.
If you already have chips
Do not panic about the shallow ones. Get the deep ones sealed so water is not sitting on primer through winter. Then decide honestly whether the front end bothers you enough to do something about, because plenty of owners look at a chipped bonnet, shrug, and drive on perfectly happily for another decade.
If you are not sure what you are looking at, bring it by. It takes about two minutes with a light and a gauge to tell you whether a mark is cosmetic or whether it needs attention, and I will tell you when the answer is that it needs nothing at all.
