· by Michael Blair, Nashville's Mobile Detailing
Paint correction takes swirls out of a car by taking a little clear coat with them. That is the whole method: the polish levels the surface down to the bottom of the marks. So the useful question before a correction in a Franklin driveway is not how good the car can look, but how much clear coat it has to give. Here is what one automaker told its own repair technicians about that limit, why we put a gauge on the paint before a pad touches it, and how the readings decide between a light polish and a full paint correction.
How much clear coat can polishing safely remove?
Very little, by one automaker’s numbers. In a 2013 collision bulletin on overspray removal, filed with the National Highway Traffic Safety Administration, Chrysler Group told technicians to remove no more than 0.2 mils of clear coat when polishing, and noted that most of its vehicles carry about 1.5 mils of clear coat. A mil is a thousandth of an inch.
The same bulletin allows up to 0.5 mils when the panel is wet sanded and then polished. That is one maker’s guidance for its own vehicles, written about removing overspray, and other makers build their paint differently, so we treat it as a reference point and not as a rule for every car. It still makes the scale plain: by those figures a polish may take about an eighth of the clear coat, and a sand-and-polish about a third.
Why measure paint thickness before a correction?
Because the clear coat you cannot see is the budget, and a gauge is the only way to read it from the outside. The Chrysler bulletin calls for measuring paint thickness with a gauge before and after the work. DeFelsko, which makes paint thickness gauges, explains on its automotive paint inspection page that a vehicle should normally read about the same thickness all over, and that differences, even small ones, can point to blending or repair work.
A gauge reads the whole paint film, not the clear coat alone. DeFelsko’s worked example shows readings around one vehicle of 4.2 to 4.5 mils, which takes in the primer and color under the clear. So we read each panel and compare it with its neighbors. A door or fender that reads well under the panels around it has either been polished hard before or repainted, and either way it gets the lightest touch on the car.
Steel and aluminum panels need different probes, and DeFelsko sells combination gauges that read both, plus an ultrasonic gauge for paint over plastic parts such as bumper covers.
Does the car need a one-step polish or a full correction?
Most cars that are dull and lightly swirled need one step, and full correction is for defects deep enough to be worth more of the clear coat. The Chrysler bulletin’s order is to begin with the least aggressive method, washing, then clay, then polish, and to go further only when that falls short. We work the same way: a test section on one panel with the mildest pad and polish that might do the job, checked under light, before the rest of the car is touched.
A full correction means more passes or a more aggressive compound, and every pass spends clear coat. The bulletin adds that a rotary buffer should be used only by a trained technician. Some marks sit deeper than the readings say the paint can spare, and when that is the case we tell you after the test section rather than chase them to the bottom.
Keeping a corrected finish
A corrected finish lasts as long as the washing allows. Most swirl marks are put in by washing, which our post on washing without swirl marks explains, and every later correction draws on the same thin layer. A careful wash is what keeps the next correction a light one.
We do corrections at the car, in a driveway or a parking space, across Franklin and the rest of Middle Tennessee. When you book a paint correction, tell us whether the car has had body work or a correction before, since both change what the gauge is likely to show.
Sources
- Chrysler Group LLC, Collision Bulletin 31-001-13, Overspray Removal (2013), filed with NHTSA — No more than 0.2 mils of clear coat removed when polishing, 0.5 mils when wet sanding and polishing; most vehicles about 1.5 mils of clear coat; least aggressive method first (wash, clay, polish); rotary buffer by a trained technician; measure with a gauge before and after. One automaker’s guidance for its own vehicles.
- DeFelsko, Automotive Paint Inspection Paint Meters — Vehicles should read about the same thickness throughout, and variations can indicate repair; example readings of 4.2 to 4.5 mils; gauges for steel, aluminum and paint over plastic.
FAQ
How Much Clear Coat Paint Correction Takes Off — questions
Does paint correction damage clear coat?
It removes some clear coat by design, because polishing levels the surface down to the bottom of the swirls. One automaker’s bulletin limits polishing to 0.2 mils out of about 1.5 mils of clear coat on most of its vehicles. Measured with a gauge and kept to the mildest method that works, a correction leaves plenty behind.
Should you clay before doing paint correction?
Yes. The Chrysler bulletin’s order is wash first, then clay, then polish, starting with the least aggressive step and going further only when it falls short. Clay lifts bonded contamination off the paint so the polishing pad is not dragging it across the clear coat, and some cars need nothing more than that.
How often should you polish your car?
As seldom as the finish allows. Every polish takes a thin layer of clear coat, and one automaker’s bulletin puts the limit for polishing at 0.2 mils. A careful wash that does not put swirls back in is what keeps the next polish years away, and a light one when it comes.
What does a paint thickness gauge show on a car?
It reads the total paint film, primer and color and clear together, in mils or micrometers. A car should read about the same all over, so a panel that reads noticeably thinner or thicker than its neighbors points to earlier polishing or repair work. We read every panel before we choose a polish.