Why bother painting supports at all
Automatic support generation in PrusaSlicer works off overhang angle. Anything steeper than your threshold gets a support column, whether it actually needs one or not. That works fine on simple shapes, but on anything with a mix of steep overhangs, bridges, and fine detail, the algorithm ends up guessing. It will happily bury a delicate surface detail under a forest of support material, or skip a small overhang that would have drooped without help.
Paint-on supports fix that by moving the decision from PrusaSlicer’s angle calculation to your own eyes. You look at the model, decide which spots actually need backup, and mark them directly on the mesh. The result is fewer supports where you do not want them, and supports exactly where you do, without spending twenty minutes fighting global overhang sliders trying to hit a compromise that never quite works.
This is not the tool for every print. If the model is simple and the default angle threshold already does a reasonable job, painting is wasted effort. Reach for it when a model has a mix of needs across the surface: a figure with an outstretched arm plus fine facial detail, a bracket with one problematic overhang and a dozen harmless ones, or anything where you already know from a failed print exactly which spot let you down.
Enforcers vs. blockers: what each one actually does
PrusaSlicer’s paint-on tool gives you two brush modes, and they solve opposite problems.
Support blockers remove supports from areas that would otherwise get them automatically. According to Prusa’s own documentation, blockers are useful when you are generally happy with the automatic supports and just want to remove some of them. The trick is to paint the blocker where the support touches the object, not just anywhere along the support column. PrusaSlicer removes material starting from that contact point, and the rest of the structure below it clears out on its own.
Support enforcers do the opposite: they force a support to generate somewhere the automatic algorithm skipped. This mode is meant for situations where only a few parts of the object need support but the auto-generation is putting columns everywhere. If you paint enforcers and want PrusaSlicer to place supports only where you painted, you need to switch the support generation mode to "For support enforcers only" in the support settings. Skip that step and the slicer will keep generating its usual automatic supports on top of whatever you painted, which defeats the purpose.
You can combine both in the same paint session. Paint blockers to strip supports off areas you know are fine, then paint enforcers on the handful of spots the algorithm missed, and set support generation to "Everywhere" or "From build plate only" so both painted layers get respected alongside the normal automatic logic.
Getting into the tool and the actual workflow
The paint-on supports tool lives in the left-side toolbar in PrusaSlicer, but only shows up once you switch to Advanced or Expert mode. Simple mode hides it along with most of the other manual controls.
Select your object, click the support-painting icon, and the view changes: other objects in the scene get hidden and your selected object turns light gray so the paint strokes stand out clearly against the surface.
From there the controls are:
- Left mouse button paints enforcer material onto the surface.
- Right mouse button paints blocker material.
- Shift plus left click erases whatever you have painted in that spot.
- Alt plus mouse wheel changes brush size on the fly, so you can widen the brush for broad areas and shrink it down for tight corners without digging into a menu.
There are also two brush behaviors worth knowing before you start clicking. The default is a straightforward Brush that paints whatever facets the cursor touches. The second is Smart fill, which paints across neighboring facets within an angle threshold you set, so one click can cover an entire curved surface instead of you dragging the brush over every inch of it. Smart fill is the faster option on rounded or organic shapes; the plain brush gives you tighter control on flat or mixed-angle surfaces.
PrusaSlicer also gives you a "Highlight overhang by angle" slider inside the tool. Turn it on and any facet steeper than your chosen angle lights up in real time, which is a genuinely useful way to spot overhangs you might have missed just by eyeballing the model. There is an option to restrict your brush strokes to only the highlighted overhang areas too, which is handy if you are painting a complex model and do not want to accidentally mark flat vertical walls.
For models where the area you need to paint is hidden inside the mesh, there is a clipping plane slider that lets you cut away part of the model’s view so you can see and paint the interior surface. A "Reset direction" button snaps that clipping plane back to whatever angle you are currently viewing the model from, which saves you from manually re-aligning it every time you rotate the camera.
Saving your work
Painted supports are not a one-off setting that lives only in memory for the current session. If you save the file as a PrusaSlicer project (3MF), all of your enforcer and blocker strokes get saved along with it. Reopen that project later and the paint is still there, ready to adjust if you change your mind or the geometry shifts slightly after an edit.
This matters more than it sounds like. If you only export a plain STL after painting, you lose that information the moment you close the project, because the paint data is not part of the STL format at all. Treat the 3MF project file as your actual save file whenever you have manual support work in it.
One more detail worth knowing: while the paint-on tool is open, PrusaSlicer tracks a separate undo and redo history just for your strokes. You can step back through individual brush actions (enforcer strokes, blocker strokes, erases) without that undo history bleeding into your general model-editing undo stack, and without losing track of what you already painted.
Common mistakes people run into
The single most common trap is forgetting to switch the support generation mode after painting enforcers. If you paint enforcer strokes but leave the support setting on its default, PrusaSlicer keeps auto-generating supports everywhere the angle threshold demands, on top of your painted ones. You end up with all the automatic clutter you were trying to avoid, plus your manual additions. Always double check that the support mode setting matches what you intended: "For support enforcers only" if you want exclusively your painted spots supported, or "Everywhere"/"From build plate only" if you are mixing painted work with automatic generation.
The second trap shows up after you edit the model. If you resize, reposition, or reshape the object after painting supports, the painted regions do not always track cleanly with heavy geometry changes. It is worth reopening the paint-on tool after any significant model edit just to confirm your strokes still line up with the surface the way you expect, rather than assuming they carried over perfectly.
Third, people paint blockers in the wrong spot and wonder why the support did not disappear. Remember that a blocker needs to sit where the support actually contacts the model, not somewhere further down the support column. Paint below that contact point and PrusaSlicer has no reason to remove anything, because as far as it is concerned nothing there was blocked in the first place.
Finally, do not skip the "Highlight overhang by angle" view before you start painting blind. It takes a few seconds to turn on and it will show you overhangs you would otherwise have missed on the back or underside of the model, the kind you only usually discover after the print has already failed.
When manual painting is worth the extra time
If you are printing the same model repeatedly and have already identified the two or three trouble spots through trial and error, painting supports once and saving the project file means every future slice starts from that known-good configuration. That is the real payoff: not a single print, but every reprint after it, without re-litigating the same overhang decisions from scratch.
For a one-off print of a simple shape, stick with automatic generation and adjust the overhang angle threshold instead. Save the paint brush for the models where automatic generation genuinely gets it wrong.
Painting also pairs well with PrusaSlicer’s organic support style: paint enforcers where you know a branch needs to land, then let the organic algorithm route the rest. And if you are not yet sure whether your model needs manual placement at all, start with the general guide to when supports are needed.