A print coming off the bed is a starting point, not a finished part. How you treat it afterwards determines whether it looks and performs like a prototype or a product.
Setting expectations by process and material
Post-processing isn't a universal workflow — it branches immediately based on what process produced the part and what material it's made from. FDM parts in PLA behave differently under abrasives than PETG, and differently again from ABS or ASA. Resin prints from an MSLA printer arrive with a smooth surface that needs very little sanding but requires careful washing and curing before anything else happens. SLS nylon parts come out with a slightly grainy, matte surface that accepts paint readily but responds poorly to aggressive sanding. Knowing which branch you're on before picking up any tool saves time and prevents the specific kind of damage that comes from applying the right technique to the wrong material.
The end use also sets the appropriate level of effort. A functional bracket that lives inside an enclosure doesn't justify the same finishing time as a client-facing prototype or a display model. Post-processing has a cost in time and consumables, and calibrating that effort to the actual requirement is as much a professional judgement as any decision made during the design phase.
Preparation: what happens before sanding
Support removal and cleanup precede everything else, and they deserve more care than they typically receive. A support contact nub torn out aggressively leaves a crater that no amount of sanding will make invisible — it needs filling before the surface can be worked flat. Remaining raft material on the base of FDM prints, thin flash lines on resin parts, and any warped or delaminated edges need to be addressed at this stage, not discovered after the primer coat is down.
For resin parts specifically, washing in isopropyl alcohol is not optional before any further work. Uncured resin on the surface will prevent paint adhesion, contaminate sandpaper within a few strokes, and is a skin irritant that becomes significantly harder to remove once it's spread across a workbench. Wash thoroughly, dry completely, and cure under UV before touching the surface with anything else. The order matters and skipping steps has compounding consequences.
Filling layer lines before sanding, rather than trying to sand them out entirely, is almost always the more efficient approach for FDM parts. Spot filler primer, two-part body filler, or UV-cure resin applied to the surface and allowed to harden gives the abrasive something to cut that isn't the part itself. Sanding across a filled surface levels the high points of the layer lines without digging into the valleys, which is the mechanic that actually produces a smooth result. Trying to sand raw FDM layer lines flat without any filler is possible with enough patience and progression through grits, but it removes material from a part that may have limited wall thickness to spare.
Sanding: progression and what each grit is actually doing
Sanding works by cutting the surface with progressively finer abrasives, each one removing the scratches left by the previous grit. Starting too fine means the abrasive can't cut fast enough to be useful. Starting too coarse means the scratches it leaves require significant work to remove at subsequent stages. For most 3D printed parts, a starting grit of 120 to 180 is appropriate for initial surface levelling — coarser if there are significant high spots or tool marks to remove first.
The progression matters more than any individual grit. A typical sequence for a part intended to be painted might run 120, 220, 400, 800, with a light intermediate coat of primer applied after 220 to reveal remaining surface defects before continuing. Priming mid-sequence is not cosmetic — it shows you exactly where the surface is still uneven in a way that dry sanding under workshop lighting does not. Skipping that reveal step means finishing and painting a surface that still has defects, which the first coat of topcoat will make fully visible.
Wet sanding from 400 grit upwards significantly improves the result and extends the life of the abrasive. Water floats the abraded material away from the surface rather than letting it clog the paper, which keeps the cutting action consistent and reduces the heat generated by friction — relevant for thermoplastics that can soften or smear if the surface temperature rises. For parts that will receive a high-gloss finish, continuing through 1000, 1500, and 2000 grit wet before painting produces a base that the topcoat can actually replicate rather than trying to compensate for.
Filling layer lines before sanding, rather than trying to sand them out entirely, is almost always the more efficient approach. Give the abrasive something to cut that isn't the part itself.
Chemical smoothing: where it works and where it doesn't
ABS and ASA can be smoothed with acetone vapour, which partially dissolves the surface and allows it to reflow into a much smoother finish without any abrasion. The results on ABS in particular can be dramatic — layer lines that would take an hour to sand out disappear in a few minutes of vapour exposure. The process requires a sealed container, careful temperature control, and adequate ventilation, and it does slightly reduce dimensional accuracy as the surface material migrates. For cosmetic parts where surface finish matters more than precise geometry, it's an efficient tool. For functional parts with critical dimensions, it's not appropriate.
PLA, PETG, and most other common FDM materials don't respond to acetone in the same way. There are solvents that attack them — methyl ethyl ketone for some formulations, ethyl acetate for PLA — but the results are less controlled and more likely to produce uneven dissolution than smooth reflow. For these materials, mechanical finishing is the more predictable route.
Resin prints can be given a secondary coating of fresh resin brushed onto the surface and cured under UV, which fills micro-porosity and surface irregularities in a single step. This technique requires thin, even application and thorough UV exposure, but it can produce a near-optical surface on a resin part before any sanding begins, dramatically reducing the mechanical work required afterwards.
Primer selection and why it matters
Adhesion primer vs filler primer
These are different products that do different things. Adhesion primer is thin, designed to bond to the substrate and provide a key for subsequent coats — it does very little to fill surface texture. Filler primer is thicker, builds up in multiple coats, and is designed to be sanded back to level the surface. For 3D printed parts with visible layer lines, filler primer is the appropriate starting point. Using adhesion primer on an unlevelled surface and expecting the topcoat to hide the texture is a reliable way to be disappointed.
Spray vs brush application
Rattle-can primer applied in thin, even coats from the correct distance is faster and produces a more uniform film thickness than brush application for most part geometries. Brush application has its place on very small parts or in areas that aerosols can't reach without overspray contamination, but it introduces brush marks that become another surface defect to remove. For anything larger than a thumbnail, aerosol or airbrush application is the more controllable method.
Compatibility with topcoats
Not all primers are compatible with all topcoats. Certain solvent-based topcoats will attack water-based primers and cause wrinkling or lifting. Check compatibility before committing to a primer, particularly if switching between water-based and solvent-based products at different stages. Testing on a scrap print costs nothing and prevents losing a finished part at the final step.
Painting and colour finishing
Painting 3D printed parts follows the same principles as painting any plastic substrate, with the added consideration that 3D printed surfaces are more porous and texturally variable than injection-moulded plastics. This means primer is not optional for any cosmetically important finish — the paint will behave differently over raw print material than over a properly prepared primed surface, and the difference will be visible in the final result.
Thin coats applied repeatedly produce better results than thick coats applied quickly. A single heavy coat traps solvent beneath the surface film, which causes bubbling, runs, and extended drying times. Three or four thin coats with proper flash-off time between them build a uniform film with good mechanical adhesion to the primer beneath. For aerosol application, the correct technique is a moving pass at consistent distance — starting and stopping the spray off the part, not on it — which avoids the heavy deposits at each end of a stroke that create runs at the edges of the part.
For parts that will be handled, a clear protective topcoat over the colour coat significantly extends the finish life. Matte, satin, and gloss variants are available, and the choice is partly aesthetic and partly functional — gloss surfaces show fingerprints readily but are harder and more chemically resistant than matte clears, which have a softer surface layer that scratches more easily. For display models and props, matte or satin clears tend to look more resolved. For functional parts that will see regular handling, gloss or semi-gloss provides better durability.
Alternatives to painting
Not every finishing requirement is best served by paint. Hydrographic printing — also called water transfer printing — applies a printed film pattern to a part surface using water tension, producing wood grain, carbon fibre, camouflage, and other complex surface patterns that would be extremely difficult to achieve with conventional painting. The process requires priming, a base colour coat compatible with the film, and a protective clear after transfer, but the surface pattern quality is higher than anything achievable by hand painting alone.
Plating — either electroplating or spray-on metallic plating products — is applicable to FDM and resin parts for applications where a metallic finish is required without the weight or cost of a machined metal part. Electroplating requires the part surface to be made conductive first, typically with a spray-on graphite or copper conductive primer, and produces a genuine metal surface with appropriate hardness and chemical resistance. Spray metallising products are simpler to apply but produce a thinner, softer metallic layer that is more decorative than functional.
For parts where dimensional accuracy at bore features matters post-finishing, it's worth accounting for the cumulative thickness of primer, colour coat, and clear before the print is complete. Multiple coats of primer and paint on an FDM part can add 0.1 to 0.3 mm to the surface — which is irrelevant on large external faces but will close up a precision bore or interfere with a snap-fit feature that was modelled to a tight tolerance. Either model the clearances with the paint thickness in mind, or mask critical features before painting and clean them up afterwards.
Post-processing is where the print becomes the part. The decisions made at this stage — what to fill, how fine to go, which primer, how many coats — have as much effect on the final result as any setting in the slicer. Treating it as an afterthought produces results that look like one.
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