A raw FDM print reads as a 3D print from across a room. Layer lines catch light in horizontal bands, support scars leave rough patches on downward faces, and the surface has a matte granularity no consumer product has. For an engineering fit check that is irrelevant. For an investor demo, a trade show table, a photo shoot, or a package mockup, it is the whole ballgame — and closing that gap is hand labor, not a printer setting.

Set the Finish Target Before You Print

Three finish levels cover almost every request. A functional finish means supports removed, sharp edges knocked down, and nothing else. A presentation finish means no visible layer lines, uniform color, and even sheen at arm's length. A show finish means the part survives macro photography and a customer turning it over in good light.

Each level roughly triples the labor of the one below it, so the target belongs in the quote before anyone starts printing. Print choices flow backward from it too: orient the cosmetic face upward or vertical rather than against supports, add 0.3 to 0.5 mm of stock on surfaces you plan to sand through, and consider whether a resin print starts you two steps ahead, a tradeoff covered in FDM versus SLA printing.

The Sanding and Filling Loop

This is the core workflow and it is iterative, not linear. You sand, you discover a valley the sanding revealed, you fill it, you sand again.

  1. Remove supports and scars. Flush cutters, then a sharp blade to shave nubs flat. Do not sand support scars away — you will dish the surrounding surface.
  2. Knock down layer lines at 180 to 220 grit on a rigid sanding block. A block matters more than the paper: fingers follow the layer contour and preserve exactly what you are trying to remove.
  3. Apply filler primer in two or three light coats. High-build automotive filler primer is the workhorse; spot putty handles deeper defects like a bridging sag or a seam gap.
  4. Wet sand at 320, then 400 grit. Water carries away swarf and keeps the paper from loading. This is where the surface becomes genuinely flat rather than merely smooth.
  5. Guide coat and repeat. A dusting of contrasting spray paint reveals lows that survive sanding. Repeat the prime-and-sand cycle until the guide coat clears uniformly. Two cycles is typical, three on organic surfaces.
  6. Refine to 600 grit for a satin paint base, or to 1,500 and 2,000 grit plus plastic polish if the part will be clear-coated to a gloss.

Vapor Smoothing and Its Limits

Solvent vapor melts the outermost few microns of a part so surface tension pulls it flat. It is fast, it needs no hand labor, and it reaches interior geometry a sanding block never will.

The catch is material. ABS and ASA respond to acetone vapor; PLA and PETG essentially do not, which quietly rules out the two most common desktop filaments. Commercial systems using proprietary solvents handle nylon and other engineering polymers, and the standard shop route for a smooth-by-default part is powder-bed nylon plus vapor treatment, described in SLS nylon printing.

Vapor smoothing also softens fine detail, rounds crisp edges, and can slightly shrink or distort thin walls. Small engraved text is often the first casualty. Run it on a sacrificial part before committing a part you cannot reprint, and treat solvent handling as a ventilation and fire question, not a convenience one.

Primer, Color, and Clear

Painting is where a part stops looking like a prototype. Prime, wet sand at 400, then apply color in three to four light coats rather than one heavy pass, holding the can eight to ten inches out and keeping it moving. Heavy coats run and they bury the crisp geometry you just spent hours creating.

Clear coat decides perceived quality more than color does. Matte clear hides residual texture and photographs forgivingly. Gloss clear photographs beautifully and betrays every flaw underneath, so only choose it on a surface you have genuinely finished. Soft-touch and rubberized topcoats read as premium in the hand but scuff badly on a demo unit that gets passed around a booth for three days — the tradeoffs are laid out in soft-touch coating. Matching a target color, gloss, and texture across several parts is a design decision in its own right, which is why CMF specification should exist on paper before the spray gun comes out.

Where the Labor Cost Actually Lands

For a hand-sized part, roughly 120 by 80 by 40 mm, US model shops bill finishing at $65 to $120 per hour. A functional cleanup runs fifteen to thirty minutes. A presentation finish runs three to five hours, so $200 to $500 a part. A show finish with guide coats, color match, and polished clear runs eight to fourteen hours, landing between $600 and $1,500.

The number that surprises people is the multiplier: the print itself might be $45 of machine time, and the finish is ten times that. Finishing five identical demo units does not cost five times one unit — it costs about four and a half, because setup and color mixing amortize — but it is still the dominant line in the prototype budget. Reducing part count, hiding parting seams at natural breaks, and choosing textured rather than glossy surfaces are the cheapest ways to cut it.

When to Stop Finishing and Start Casting

Past roughly eight to ten identical cosmetic units, hand finishing stops making sense. Finish one master perfectly, pull a silicone tool from it, and cast the rest in pigmented urethane. Every copy then arrives with a molded surface that needs only light cleanup, and the per-unit finishing hours collapse. That crossover, and the tooling economics behind it, are worked through in urethane casting for low-volume production. It is also the honest answer when a founder asks for thirty show-quality units in two weeks.

Getting Parts That Look Like Product

Projects House builds presentation and show-finish models for US product ventures through a global model-making network, with the finish level, color target, and photo requirements agreed before the first print starts. Send your CAD file, your intended use for the parts, and your deadline through our contact form.