Smoothing Methods and What They Change

Every method changes the part

Smoothing is never only cosmetic. Separate what it does to looks from what it does to size, detail and strength before you pick a method:

Method	What happens to the surface	Dimensions and detail	Strength	Lesson
Sanding	Removes material	Part gets smaller, edges round, fine detail goes first	Thin walls can be sanded through	Sanding
Filler and filler primer	Adds material	Part gets thicker with every coat; per-coat film thickness is in Filling and Priming	Filler primer: surface only; body filler can reinforce seams (Polymaker)	Filling and priming
Acetone vapor (ABS, ASA)	Softens and reflows the outer surface	Prusa: can change dimensions and shapes, especially sharp edges; tiny structures lose detail or deform; thin parts can twist	Prusa measured changes both ways (see below)	This page
Alcohol (IPA) on PVB filaments	Softens the surface	Not quantified by Polymaker	Polymaker: sacrifices mechanical strength and heat resistance	This page; IPA is flammable too (see below)

For parts that must fit, keep every one of these off mating faces, or plan to re-measure and adjust. See Fits and Tolerances.

Which plastics can be solvent-smoothed

• ABS and ASA dissolve in acetone. Prusa and Polymaker both describe acetone vapor smoothing for them, giving a glossy surface. See ABS and ASA.
• PVB filaments (sold for alcohol smoothing) soften in isopropyl alcohol. Polymaker recommends this for cosmetic models and display pieces and advises against it for functional parts. It does not remove the fire risk: Polymaker says "IPA is less flammable than acetone but still volatile. Avoid heating above 40°C." Maker spec IPA handling and storage are covered in Washing and Curing Resin Prints.
• HIPS: Prusa found some HIPS also smoothed in acetone fumes; it washed other HIPS in d-limonene.
• PLA: in Prusa's tests, acetone bonded PLA parts but did not smooth them; hours in vapor only softened the parts, and a full bath bleached some filaments and cracked every model. Prusa could not obtain the solvent often suggested for PLA, and advises against sodium hydroxide, which will most likely break the part into fragments. See PLA.
• PETG: Prusa's tests used dichloromethane, which it calls dangerous and restricted. That is not a home process. See PETG.

For PLA and PETG, the practical smoothing methods are sanding and filler primer.

Acetone hazards, from the SDS

This guide gives no vapor-smoothing procedure. Both maker procedures cited here heat the acetone: Polymaker's warms the container on a hot plate or build plate, and Prusa's heats the acetone bath. Read the SDS for the acetone you buy before you try either. Fisher Scientific's lab-grade acetone SDS, as an example, says:

• Highly flammable liquid and vapor, with a flash point of −20 °C (−4 °F) Maker spec. It gives off ignitable vapor well below room temperature.
• Vapors may form explosive mixtures with air and may travel to a source of ignition and flash back. Containers may explode when heated.
• Keep it away from heat, hot surfaces, sparks, open flames and other ignition sources; no smoking. Use explosion-proof electrical, ventilating and lighting equipment, non-sparking tools, and take precautions against static discharge.
• Use only outdoors or in a well-ventilated area. It causes serious eye irritation and may cause drowsiness or dizziness.
• Wear gloves, protective clothing and eye or face protection, with an approved respirator if exposure limits are exceeded or irritation occurs.

The SDS for a hardware-store acetone, Klean-Strip Acetone from W. M. Barr, adds warnings written for home use:

• Vapors are heavier than air and may spread along floors, and may cause flash fire or ignite explosively.
• Do not use it near any source of heat or open flame, furnace areas, pilot lights or stoves.
• Do not use it in small enclosed spaces such as basements and bathrooms, where vapors can accumulate and explode if ignited.
• A dust mask does not protect against vapors. Where ventilation is inadequate, it calls for a NIOSH-approved respirator for organic solvent vapors.
• Splash goggles for eye protection.

The makers add their own warnings:

• Polymaker: acetone is highly flammable; smooth only in well-ventilated areas away from open flames. It names non-heated methods such as cold vapor baths as lower fire risk.
• Prusa: the most common and the safest process is placing napkins or paper soaked with acetone under and around the model, without heating; safest of all, it says, is a professional smoothing station. "If you are not sure about it, use the cold method."
• Prusa: heating acetone is dangerous and at your own risk. A smoothing container must never be perfectly sealed, so pressure cannot build. Any heater belongs outside the container, and a damaged fan can spark and ignite the fumes. Prusa's own test box used a printer heatbed and a computer fan; neither is the explosion-proof electrical and ventilating equipment the acetone SDS calls for. Never leave it unattended. Wear gloves and a face shield or glasses, and work far from flammable objects.
• Prusa notes that acetone vapor is very noticeable and may cause respiratory problems, and that the smell can last for days.

Hot plates and heated beds are hot surfaces, which the SDS lists among ignition sources. Fire safety around printers is covered in Fire and Electrical Safety.

What vapor smoothing does to the part

• Shape and size. Prusa found smoothing can change dimensions and shapes, especially sharp edges. Large flat models can deform and thin parts can twist. Polymaker warns of deformation with prolonged exposure and suggests several short passes over one long one.
• Uneven results. Polymaker notes that acetone vapor sinks, so lower sections can be over-smoothed.
• Soft for hours. Prusa says smoothed models stay soft for hours and can be damaged by touch.
• Cracking. Polymaker says excessive acetone contact may cause long-term cracking.
• Strength. Prusa's article calls smoothing aesthetic only, yet its own tests found slightly better impact toughness, more than double the layer adhesion, and slightly lower tensile force at break. Treat those as Prusa's results for its samples, not a promise for your part.
• Water resistance. Polymaker says acetone smoothing increases water resistance and reduces porosity on ABS and ASA.

Choosing a method

1. Functional or fitted part: smooth only the faces that do not mate, by sanding. Leave tolerances alone.
2. Display piece in PLA or PETG: sanding, then filler primer, then paint. See Filling and Priming and Painting 3D Prints.
3. Display piece in ABS or ASA: sanding and filler primer work here too (aerosol primers carry their own fire warnings). Acetone vapor is an option only with the SDS precautions above and an accepted loss of sharp detail; if you are not sure, use the unheated method or skip it.
4. PVB filament: alcohol smoothing, for cosmetic parts only, following the filament maker's instructions and the IPA's SDS, unheated or below Polymaker's 40°C limit, away from ignition sources.