5 signs a scratch is too deep to polish out

You get back from the supermarket and there it is: a fine, pale line running right across the driver's door. A trolley, a branch, a rucksack, who knows. The first question is always the same: can I get this out myself with a polish, or does the car need to go to a body shop? If you just start polishing away, the worst case is that you lose clear coat and keep the scratch anyway.

Whether you can polish out a scratch comes down to how deep it is compared with the clear coat. Five signs tell you before the first pass whether hand polishing will do it, or whether the scratch goes down into the primer and needs a paint repair.

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1. Your fingernail catches in the groove

If your fingernail noticeably catches as you drag it across the scratch, the groove usually goes through clear coat that's only 30 to 50 micrometres thick, and a polish can at best tone it down.

When you polish out scratches, what you're really doing is taking down the intact clear coat around them until the surface sits level with the bottom of the groove. The scratch disappears because its edges disappear. That only works as long as there's still enough clear coat left under the bottom of the groove.

A quick look at how paint is built up puts this in context. From the factory, the first layer on the bare metal is the e-coat at 15 to 25 micrometres, then the primer (filler) at 25 to 40, then the colour-giving base coat at just 10 to 20, and the clear coat on top. With European manufacturers that usually adds up to 80 to 150 micrometres. The only layer you're allowed to polish is the top one, and even there only a small part of it.

Still, the fingernail test isn't guesswork. It's the first check Dr. Wack itself names in the product information for A1 "Kratzer Polish": if you can still feel the scratch with your fingernail, it's usually already too deep. Do it properly and it tells you a lot. The spot is washed and dry, your nail sits at roughly a right angle and glides across the line with light pressure, not along it.

If your nail slides straight over, the scratch is very likely sitting in the clear coat. That's exactly the job polishes are made for. You can polish the scratch out without burning through your reserve. If it catches noticeably, this is where you stop. The test does have one limit: on very soft solid paint, a sensitive nail will also pick up grooves you can still polish out. That's why the next four signs come into it before you decide.

2. The scratch stays visible when the area is wet

If a scratch disappears while the area is wet, it's very likely only in the clear coat. If it's still there as a line under water, the colour layer underneath has usually already been damaged.

It comes down to optics. A scratch in the clear coat has no colour of its own. You only see it because its rough walls scatter light in every direction, which is why it looks white or silvery. Clear coat refracts light with a value of about 1.5, water about 1.33 and air 1.0. Once water fills the groove, the difference to the paint is suddenly small, the scattering collapses and the line goes almost invisible for a moment.

Here's how you do it: wash the spot, wet it with plain water, no shampoo, no gloss enhancer and no wax, and then look at it from several angles. If the line almost completely disappears, that's a good sign. If it comes back as it dries, that's normal. It was never gone, the water just bridged it optically.

If, on the other hand, it stays clearly visible while wet, as a dark line on light paint or a light line on dark paint, the scratch has broken through the clear coat. No polish takes off enough material to make it disappear, because the entire clear coat around it would have to come down to the depth of the groove. This check costs you two minutes and saves you an hour of work on a spot you can't save.

3. A different colour shows through in the scratch

If you can see grey, matt white, black or bare metal at the bottom of the scratch, it has gone through the clear coat and the base coat, and no polish can replace the missing colour layer.

For this check, grab a magnifying glass or your phone camera with zoom, plus a small torch. A scratch that only sits in the clear coat shows your car's colour at the bottom, just lighter and duller. If the line is darker than the paint around it, the base coat has usually been hit. Grey or matt white points to the filler, in other words the primer, while shiny silver or even brownish rust means you're down to the metal.

From here on, taking material off won't help, because the problem is material that's missing. Small, deep scratches down to bare metal need sealing promptly with a touch-up pen, so that moisture and winter road salt don't push rust under the paint. It rarely looks great, because paint from a pen builds up thicker than sprayed paint. A spot like that only looks clean again after a spot repair or a respray.

The flip side deserves an honest answer too. Dr. Wack itself states that scratches reaching the primer are only toned down by A1 "Kratzer Polish". The polish rounds off the sharp edges of the groove, so it scatters less light and stands out less in everyday use. The scratch isn't gone, though, and at a lease return a spot like that still counts as paint damage.

4. After two passes the inspection spray shows the same edge

If the scratch is unchanged after two clean passes and a wipe-down with an inspection spray, it's deeper than hand polishing can take down in a reasonable amount of time.

A test section is the most honest diagnosis when the first three signs don't give you a clear answer. With A1 "Kratzer Polish", the manufacturer's method goes like this: shake the tube, put a dot about one centimetre across onto a slightly damp microfibre cloth, then work it in with plenty of pressure in small circles for up to one minute. Let it dry on, then buff it off with the dry side of the cloth. Do all of this at 20 to 25 degrees, in the shade and on cool paint.

A single scratch on a door makes the ideal test area. You're working on an area about the size of your palm, the rest of the panel stays untouched, and you can compare the result directly with the paint next to it. That way you can polish out the scratch without taking on the whole door, and you find out early whether the edge is moving at all.

Then comes the step a lot of people skip. Many polishes and almost all DIY-store scratch removers contain oils or silicone that settle into the groove and fill it. It looks perfect in the garage and is back after three washes. Panel Preparation Spray "Pps" from Koch-Chemie is an alcohol-based degreaser that strips those residues and shows you what has really happened to the scratch.

That's exactly how AUTO BILD tested ten scratch removers in 2022: after polishing, the test panels were wiped down with an alcohol solution and rated again. With some products the scratch was back almost unchanged. A1 "Kratzer Polish" held its gloss and won with 259.5 out of 300 points and a grade of 1.6 (on the German 1–6 scale, 1 = best). What matters for you is the second finding from the same test: none of the products removed deep scratches down to the top coat or the primer.

That gives you a simple stop rule. If the edge looks visibly softer after the first pass, repeat once and check again with inspection spray. If it's as sharp after the second pass as before, you stop. Hand polishing takes off well under one micrometre per pass, a DA polisher with a medium polish one to two. A scratch that doesn't move after two rounds just eats clear coat with every extra round.

5. The scratch runs over an edge or body crease

If a scratch sits on a body edge, a body crease or a seam, the clear coat there is thinner from the factory, and even a medium scratch reaches the colour layer faster there than on the flat of the panel.

During painting, surface tension makes the still-liquid clear coat pull back slightly from sharp edges before it cures. On the flat, the door might have 40 micrometres of clear coat, noticeably less on the edge above it. A scratch that would be harmless in the middle of the door is therefore often already in the base coat at the edge, even if it looks the same.

Then there's the tool. On an edge, every pad and every cloth only makes contact over a few square millimetres, the pressure concentrates and the edge can barely shed heat. With a machine, that's the classic spot where people burn through the paint. That's why pros tape off edges before any machine polishing and only work up to just short of them. By hand the risk is smaller, but not zero, especially if you lean on it hard.

Be just as careful on painted plastic parts such as the bumper. Plastic is poor at shedding heat, so the paint on it heats up faster under friction than on metal. If you're not sure whether a part has been repainted before, a paint thickness gauge at a body shop or detailer will tell you in seconds. Readings between 200 and 300 micrometres point to a repair respray, and that behaves differently under polishing than factory paint.

Three products cover every scratch in the clear coat

If all five signs show green, you can get a single clear coat scratch out with three things: a silicone-free scratch polish, an inspection spray and a clean cloth for buffing off.

Dr. Wack built A1 "Kratzer Polish" for exactly this job: automatic car wash scratches, fine scratches from bushes, scratches in door handle cups and on painted bumpers, and according to the manufacturer also on hard plastic headlights. The 50 ml tube goes a long way across lots of individual spots, because you only need a dot about one centimetre across per section. Its limit is spelled out above: it cuts rather than fills, so a scratch down to the primer stays visible. Keep it off matt paint, matt wrap film and chrome.

Panel Preparation Spray "Pps" is your referee after every pass. A 500 ml bottle lasts for lots of checks, because two or three sprays per test section are enough. It doesn't clean and it doesn't polish. It strips oils and polish residue so you can see what the polish has really achieved, and it preps the surface for a wax or sealant.

For buffing off you want a soft, edgeless cloth, so the final wipe doesn't leave new marks. A1 "DAS TUCH" Premium Microfibre measures 40 by 40 centimetres, has a deep pile on both sides and is ultrasonically cut, with no stitched seam on the edge. It isn't a polishing tool, it's the second cloth alongside the one that comes with the polish: one slightly damp for working it in, one dry for buffing off. Wash it once at 60 degrees without fabric softener before first use.

If you're planning to polish at the weekend anyway, do the diagnosis this evening. Fingernail, water and magnifier take less than ten minutes all together. By Saturday you'll know which spots you'll do yourself and which ones you'll save up to show the body shop.

How much clear coat you have to work with over the years is covered in detail in Polishing out scratches: how much clear coat you actually have. How to light up your paint properly with a lamp is explained in 5 lighting mistakes that hide holograms in your paint.

Detailing1-Insight: Scratches often reach us as photos, and on a photo they nearly all look the same. What makes the difference is the order you work in on the car: fingernail first, then the wet spot, then the magnifier, and only after that the tube. If you start with the polish, you'll often spend ten minutes working a key scratch that goes down to the primer and then think the glossy, rounded-off groove is almost gone. After the inspection spray, it's back. Two passes, then decide, and the paint keeps its reserve.