TL;DR: Primer for 3D prints does two jobs, and neither of them is color. It fills layer lines so paint does not settle into the grooves, and it gives acrylic something to grip on slick PLA. If you are priming a grey print only because you want it red, a multi-filament printer can do that part instead. Below: which primer fills, how to sand it back, and where the prime-and-paint chain stops being worth the evenings.
What does primer for 3D prints actually do?
Primer for 3D prints solves two problems that have nothing to do with color. The first is texture. Every FDM layer leaves a ridge, and paint brushed straight over that ridge makes it more obvious, not less. A high-build filler primer sits thick enough to bridge the groove, so sanding it back leaves something close to flat. The second is adhesion. PLA and PETG are slick, and acrylic peels off bare plastic at the first knock. Primer gives the paint a key. Coloring the model is not on that list.
Most people reach for a rattle can for the wrong reason. They print in whatever filament was already loaded, decide the grey looks unfinished, and start a four-stage process to fix a problem that started at the slicer.
Filler primer and plain primer are not the same can
A standard automotive or hobby primer lays down a film of maybe 10 to 20 microns. A 0.2 mm layer height leaves grooves 200 microns deep. The math does not work, which is why people come away saying primer did nothing for their layer lines. What you want is labeled filler primer or high build. It carries far more solids and is designed to be sanded back rather than painted over.
Two or three light coats beat one heavy one. A heavy coat runs, pools in recesses and takes the crisp edges off any detail you printed. Let each coat flash off, then wet sand from around 500 grit up through 1000 and 2000, checking with a raking light rather than your fingertips. Prusa's guide to sanding FDM prints walks through the grit sequence, and it is worth reading before you take sandpaper to something you spent nine hours printing.
The part a primer for 3D prints cannot fix
Primer comes in grey, white, black and occasionally oxide red. Whatever you wanted the model to be, you are still painting it afterward. On a single figure that is fine. On a squad of ten, or a set of parts where each piece needs three flat areas in three different colors, the basecoating alone eats more time than the printing did.
When can you skip primer for 3D prints entirely?
You can skip primer for 3D prints when the color you want is already in the plastic and the surface finish is good enough for the job. A tabletop figure seen at arm's length does not need a filled and sanded surface, and neither does a desk toy. Print each area in its own filament and there is nothing to basecoat, so there is nothing to prime. Keep the primer for pieces that need a genuinely smooth painted finish, a gloss coat, or anything a viewer will hold up close.
Getting the colors into the model is the part that used to need CAD. It does not now. Load an STL, OBJ or 3MF into Layerpaint in the browser, use Magic Fill to flood a region with the active color, and export a Standard 3MF. The slicer reads the color groups straight in and you map them to AMS or MMU slots. Bambu Lab's multi-color printing documentation covers the slicer end of that.
Two details that trip people up. The order you add chips to the palette is the slot order the slicer sees, so add them in the order you want to load the spools. And the Match to filaments button rewrites every chip to the nearest real filament from the library, which is what keeps a color you picked on screen from being one nobody sells. If you want the longer version of that workflow, painting 3D prints without hand-painting covers it end to end.
What still needs a brush
Plenty. Filament color is flat and opaque. That is what a basecoat should be. It is the opposite of what shading needs. Eyes, washes into recesses, edge highlights, weathering and true metallics still want a brush. What changes is the ratio. Instead of three sittings of flat coverage before you touch a detail, you start at the detail. Painting miniatures by printing the base colors goes through which parts survive that split.
Layer lines do not go away either. Printing the color does nothing for surface finish, so a finer layer height and a sensible orientation still matter. A wash helps here, since it darkens the lines into something that reads as shadow. If you want to check which surfaces will even be visible before committing, drop the file into the STL viewer and rotate it.
Common questions
Does primer for 3D prints hide layer lines on its own?
Only filler primer comes close, and only with sanding between coats. A thin primer coats the ridge and follows its shape, so the lines are still there under paint. Budget for two or three coats and a wet sand if a smooth finish is the goal.
Can you paint PLA without primer?
You can, and acrylic will cover. It will also chip off any edge that gets handled, because there is nothing keying it to the plastic. For a shelf piece that never moves it is survivable. For anything picked up, primer or a sealing coat is doing real work.
What grit should I sand primer with?
Start at 400 to 500 grit to knock the high spots off the filler coat, then move up through 1000 and finish around 2000 if you are going for gloss. Wet sanding keeps the paper from clogging and stops heat building up enough to smear PLA.
Does a multi-color print still need sealing?
For anything handled, yes. A matte or satin clear coat protects the surface and evens out the sheen difference between filament brands, which can be noticeable where two colors meet. That is one coat, not a primer, basecoat and paint chain.
What file do I give the slicer?
A 3MF. It is the only common format that carries per-triangle color assignment, which is why the export is a 3MF and not an STL, and the color data lives in the Materials Extension of the 3MF specification.
Try it now
Drop an STL on the painter and color it in the browser. No account, no install, nothing uploaded. Your first export is free, then $6.97 for ten or $39.97 for unlimited.