How do I prepare a car for polishing?
Quick answer: Wash the car thoroughly with a proper car shampoo (clean bucket, fresh solution) and hit the missed traps: under mirrors and number plate, arches, sills, door shuts and roof rails. Dry off, then feel the paint with a dry microfibre. If it's rough, consider a light clay bar; light tar or sap often comes off with the polish itself. If you can see gritty black or orange particles (heavy industrial fallout); speak to a professional.
The polish gets the credit, but the prep is what decides whether you end up with a deeper shine or a panel covered in fresh marks you put there yourself.
There is no magic to this; you just need to get the car genuinely clean. People often rush the wash, and then, because they are taking great care with the polish, they suddenly notice every spot they missed. Those missed spots tend to be exactly where dirt and grit collect. What happens next is that you drag your polishing sponge through the grit and start working it all over the paint, inducing swirl marks and scratches that weren't there before. Prep is simply the work of removing everything the pad could otherwise pick up and drag.
Why a clean panel is not the same as a contaminant-free panel
This is the part most DIY guides skip, and it's the part that matters most before polishing. A car can look spotless after a good wash and still be carrying contamination that washing cannot touch. Brake dust throws tiny iron particles onto the lower panels and wheel arches; those particles bed into the clear coat and bond there. Road tar flicks up from hot tarmac and welds itself to the lower doors and sills. Tree sap drips on and bakes hard in the sun. None of it comes off with shampoo and a mitt, because none of it is sitting on the surface any more; it's gripping it.
That distinction is the whole reason prep exists as a separate stage. A wash removes the loose dirt that would scratch the paint while you work. Decontamination removes the bonded dirt that the polishing pad would otherwise grind across the panel. Skip the second step and you turn the most careful part of the job into the most damaging.
The spots almost everyone misses
A foam kneeling pad from a garden centre helps enormously; many of the traps are low down and invisible while standing next to the car. A step to stand on is useful for checking the roof too. The specific spots to check are under the wing mirrors, around the rear number plate, inside the wheel arches, under the bumpers, sills and valance, inside the door shuts, and under any roof rails or strips.
These are the same edges and recesses where Tom, our operations manager, finds the worst of the bonded contamination when a car comes in for correction. The owner has washed the visible flat panels diligently and left the lower rear quarter (the bit nearest the back wheel, straight in the brake-dust firing line) almost untouched. That lower few inches is where a microfibre will turn orange first, and it's precisely where a careless polishing pass picks up grit and carries it up onto the door.
We'd recommend using a traffic brush to help get into all those gaps, not any old brush, this is a specific soft-bristled type designed not to mar the paint.
How to wash it properly
There's a lot of advice online about how to wash a car, much of it written by enthusiasts who are fastidious about avoiding wash marks. They love cleaning cars; you probably don't, and that's fine. There are simpler methods that don't involve grit guards and the two-bucket method, and you don't actually need any of that to get the same results for a one-off pre-polish wash.
Get a bucket, use a proper car shampoo (not washing-up liquid, which strips and dries the paint), don't let the water get too dirty, and be aware that grit sinks to the bottom, so if you let your sponge or wash mitt drop to the bottom of the bucket, it will pick up that grit and put it straight back on the car. Rinse the panel before you touch it, work top to bottom so the dirtiest areas come last, and change the water the moment it looks murky.
There's also a taboo about sponges in the enthusiast community because they're hard and flat and can trap grit against the paint. A wash mitt and/or a traffic brush is preferable without question, but if all you have is a sponge, don't worry about it too much. If you're polishing precisely because you already have wash marks, switching to a wash mitt now is shutting the door after the horse has bolted; the polish will deal with the existing marks. Get a mitt for next time, once the car is polished and you want to keep it that way.
The feel test: how to read the paint after washing
Once the car is washed and dried, run your hand over the paintwork or drag a dry microfibre cloth across the panels. A clean plastic sandwich bag over your fingertips works even better: it amplifies the slightest roughness so you can feel bonded contamination your bare hand would miss. A smooth, glassy surface is ready to polish. A rough, gritty or bumpy surface needs decontaminating first, and that step matters more than most people expect.
Iron particles from brake dust embed in the clear coat and don't wash off. They look like tiny black or orange dots and stay bonded to the surface no matter how hard you scrub. If you run a polishing compound over that surface without dealing with them, the pad picks up the particles and drags them across the panel, turning prep into marring. Tar and tree sap behave the same way: the wash won't shift them, so they end up smeared and spread by the compound rather than removed.
Clay bar, iron remover and the order to do them in
For light tar or tree sap, the polish itself often has enough solvent to lift it; try polishing a test area first and inspect. If the surface still feels rough afterwards, or feels rough before you start, a clay bar is the next step. Clay glides over the paint on a lubricant and physically shears off bonded contamination (iron, tar, sap, overspray) that washing can't reach. Keep the surface wet with lube, work in straight lines rather than circles, and fold the clay to a fresh face often. Afterwards the surface will feel noticeably smoother, almost slick.
For heavier iron contamination (the orange or reddish particles that turn a microfibre rust-coloured), an iron fallout remover spray is a cleaner first move. It reacts chemically with the embedded iron and bleeds purple as it works, so you can see exactly where the contamination is sitting. Spray it on, let it dwell without drying, rinse it off, then re-do the feel test to decide whether you still need a clay bar pass.
The order matters: iron remover first to dissolve what it can chemically, then clay to shear off whatever is left mechanically. That combination (iron remover first, clay bar to finish) is the standard pre-correction sequence we run in the workshop before any machine touches the paint. Doing clay first just means the clay wears out faster picking up iron a spray would have dissolved in two minutes.
Inspect under proper light before you commit
The last prep step is assessment, and it's the one a workshop never skips. Once the paint is clean and decontaminated, get it under strong, raking light: direct sun, a garage strip light at an angle, or an LED inspection torch held nearly parallel to the panel. That low angle is what makes swirls, scratches and water spots jump out so you can see what actually needs correcting and how hard you'll need to work. Polishing blind, under flat overcast light, is how people either miss the defects entirely or chase them far harder than the paint can safely take.
When to stop and call a professional
If the paint is heavily contaminated across multiple panels with visible black or orange grit, that points to industrial fallout that is beyond a DIY clay bar: the kind a car picks up parked near a railway, a building site or an industrial estate. Speak to a professional about proper decontamination before polishing; abrasive work on heavily contaminated paint will make things worse, not better, grinding hardened particles into the clear coat with every pass.
Be honest with yourself about the whole job, too. Proper prep is a thorough wash, an iron fallout remover, tar removal, a clay bar to lift what's bonded, then a careful inspection under good lighting before a single drop of compound goes on. That is several hours before the polishing even begins, and the polishing itself is where the real skill and risk sit. A DIYer who works through this list often concludes the prep alone is more bother than it's worth, which is a perfectly fair conclusion to reach. For the full pre-polish assessment, see the car polishing preparation overview.