Guides · Filament or resin · Finishing
How to paint a 3D print so it doesn't peel
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- Paint doesn't hide layer lines and it won't stick to a shiny surface, so all the work happens before the colour does.
- Sand to about P400, wash the part, dry it at room temperature, then filler primer sanded back until the lines are gone.
- Colour goes on in two or more light coats, never one thick one.
- Recoat within 1 hour or after 48 hours, because in between is when spray paint wrinkles, and spray between 10 and 32°C.
Two things about paint, and if you take both on board the rest of this page is just detail.
Paint follows the surface it’s put on. It doesn’t fill, it doesn’t hide and it doesn’t smooth anything. A layer line with red paint on it is a red layer line. How a painted print looks was decided before the colour went anywhere near it.
Paint sticks to scratches, not to shine. A gloss surface has to be roughened up before anything will hold onto it, and a smooth printed face is exactly that kind of surface. Paint laid over a polished part has nothing to grip, and it comes off later in sheets, from a corner, usually the first time you pick the thing up.
So the order is prepare, prime, then paint.
Getting the surface right
Files for the bumps, then wet-and-dry paper up to about P400 and no further. You want it flat, not polished, because primer needs something to bite into. The method is in how to sand a 3D print smooth.
Then wash it. All the dirt, grease and oil has to come off, followed by a rinse in clean water, and a printed part you’ve been handling for an hour has plenty of all three on it. Warm water, washing-up liquid, soft brush, rinse, and let it dry all the way through.
Dry it at room temperature. PLA starts to soften under load at 55°C, so a heat gun, a radiator or a sunny car will warp the part you just spent an hour on. Time is the only safe dryer.
What filler primer is actually doing
Filler primer is spray primer with enough solid in it to fill scratches rather than just coat them, and it’s the only thing that really fixes layer lines on a curved or sloped face. Sanding can’t, because on a curve the lines are the surface stepping round the shape, so knocking the tops off just leaves it wavy.
The primer works because of where it sits. Spray a coat, let it dry, then sand it back with P400. It’s thickest in the dips and thinnest on the high spots, so the paper takes it off the peaks and leaves it in the valleys, and every round levels the surface instead of following it. Two or three rounds and the lines are gone. On a flat face, one is often enough.
Use grey if you get the choice. A single flat grey coat shows every scratch, nub and low spot you missed, which bare plastic and its layer lines hide completely. Most people’s first primer coat is a nasty surprise, and that’s the primer doing its job rather than ruining the part.
Spraying the colour
This is the part that goes wrong, so here it is exactly. The numbers are for an ordinary acrylic spray, and the back of your tin is the final word if it disagrees.
- Shake the can for a full minute after the ball starts rattling, and shake it again while you work.
- Hold it 25 to 40cm away, keep it the same distance, and keep it moving. Spraying close up is how you get runs.
- Two or more light coats, a few minutes apart. Not one coat that covers. The first coat looking patchy and thin is correct.
- Recoat within 1 hour or after 48 hours. This is the one that catches everybody. In between, the first coat has dried on the surface while still shrinking and still letting solvent out underneath, so a fresh wet coat on top drags that skin into ridges. Either carry on while it’s fresh, or walk away for two days.
- 10 to 32°C, humidity under 65%. Worth reading twice if you were planning this in a shed in November. Cold paint goes on thick and stays soft for days, and damp air can leave the finish cloudy. Indoors with a window open and a mask on beats a cold garage.
- 20 minutes to touch dry, an hour to handle, 24 hours to fully dry, measured at 21°C and 50% humidity, longer when it’s colder. And the line that matters for anything you’ll actually use: on plastic, full adhesion and durability take 5 to 7 days. So don’t judge how tough it is on day one, and don’t put it in a pocket.
A clear coat on top, matt or gloss, evens out the sheen and takes the scuffs instead of your colour. Same rules, same timings.
Why paint peels, most likely first
- Nothing to grip. The surface was too smooth, or you primed a part you hadn’t sanded.
- It was dirty. Finger grease, sanding slurry that never got rinsed off, or a part still damp inside a recess.
- One thick coat. Thick paint shrinks as it cures and pulls away from the plastic, and it stays soft underneath for ages.
- Recoated in the dead zone, between an hour and two days, so the coats never properly bonded to each other.
- Too cold or too damp when you sprayed.
- Handled too early. It’s a week to full hardness, not a day.
None of those are the paint’s fault, which is the annoying part.
Painting resin prints
Easier than filament, with one condition: cure the print properly first. Paint won’t stick to a surface that’s still slightly tacky, and a tacky resin print is nearly always one that wasn’t washed properly rather than one that wasn’t cured enough, so re-wash it before you reach for the lamp. The full order is in resin print post-processing.
Once it’s washed and cured, resin takes primer better than either PLA or PETG, because it’s an even surface with no layer lines worth filling. You still want to file the support nubs level first.
Brushing instead of spraying
For miniatures and anything with separate colours, spray the primer and brush the colour on. Acrylics, thinned a little with water, built up over several passes, because one thick coat loses the fine detail you printed the thing for. Prime even when you’re brushing. Everything above about giving the paint something to grip applies the same.
What it takes
Wet-and-dry paper, a can of filler primer, a can of colour, and somewhere ventilated that’s within the temperature range on the tin. The hour of sanding before the first coat does more for the result than any upgrade in paint.
Full disclosure: I sell a kit with the files and sanding sponges for the prep, and no paint. If you’re only ever painting things, the files and the two coarse grades are the parts of it you’ll use.
Quick questions
Do you need primer on a 3D print?
Yes, and it does two separate jobs. It gives the colour something to grip, and filler primer fills the layer lines that colour on its own would just follow. It's also the best inspection tool there is, because one flat grey coat shows every scratch and low spot that bare plastic hides.
What paint works on PLA?
Ordinary acrylic spray or brush paint, over primer. Once it's primed, PLA doesn't much care what goes on top. What it does care about is heat. PLA starts softening under load at 55°C, so don't try to speed up drying with a heat gun, a radiator or a hot car.
Why did my spray paint wrinkle?
You recoated at the wrong time. The next coat goes on within 1 hour or after 48 hours, and nowhere in between. In that gap the first coat has dried on top but is still shrinking and still letting solvent out underneath, so a fresh wet coat drags that skin into ridges. The other cause is one thick coat instead of two light ones.
Can you paint a 3D print without sanding it?
You can, and it will look like a painted 3D print. Paint follows the surface it's on, so every layer line and support mark stays visible with a colour over it. If the part is functional or nobody's looking closely, skip the sanding and spray it. If you want it to look moulded, the sanding is the job and the painting is the easy bit.
How long before you can handle a painted print?
Twenty minutes to touch dry, an hour to handle, 24 hours to dry all the way through, measured at 21°C and 50% humidity and longer when it's colder. Full adhesion and durability on plastic take 5 to 7 days. So it's usable the next day and not really tough for a week.
Can you spray paint in a cold shed?
Not below about 10°C, no. You want 10 to 32°C and humidity under 65%, and a cold damp Scottish shed is outside both. Cold paint goes on thick and stays soft, and damp air can leave the finish cloudy. Spray indoors with a window open and a mask on, or wait for a dry day.
The sanding comes first
Ten needle files and four grades of sanding sponge, which is the part that decides what the paint looks like. Posted from Scotland, UK delivery included.
Where this comes from
Every printer and material is a bit different, so treat settings here as a starting point. If something's wrong or you've got a better way, tell me.