What acetone vapor smoothing actually does
Acetone vapor smoothing works because acetone dissolves ABS and ASA on a molecular level. Instead of soaking the part in liquid acetone, you expose it to acetone vapor inside a sealed chamber. The vapor lands on the surface, liquefies the outermost layer just enough for it to flow, and the print re-hardens a few minutes later with the layer lines melted into each other. The result looks close to an injection-molded part: glossy and continuous, with the ridges of individual print layers gone rather than sanded down.
This only works on the right plastics. ABS and ASA respond well because acetone breaks down their polymer structure the same way. PLA does not react the same way at all: instead of smoothing, the surface turns sticky and rubbery, and it never recovers a clean finish. PETG is also a poor candidate. If you print mostly in PLA or PETG, this technique is not for your parts, and reaching for acetone on them will just ruin a print rather than improve it. A brush-on epoxy like XTC-3D gets PLA prints most of the way to the same glossy result without a vapor chamber.
Building a cold vapor chamber
Most home setups use the cold vapor method, which is safer and slower than heating the chamber. You need a glass or polypropylene container with a lid; acetone dissolves many other plastics, so a random food container from the kitchen drawer is a real risk. Leave roughly 30mm of clearance around the part on all sides.
Soak a few sheets of paper towel in acetone until they are damp all the way through, not dripping, and line the inside walls of the container with them. The part itself needs to sit elevated, at least 10mm above the towels, so it never touches the liquid acetone directly. A small platform, an upturned cup, or a printed stand works fine. Close the lid loosely rather than sealing it airtight, and leave two or three small vent holes a few millimeters across. A fully sealed container full of acetone vapor is a pressure and flammability problem you do not want.
How long to leave it
Timing depends mostly on part size. Small parts under about 50mm can be done in 10 to 20 minutes. Medium parts in the 50 to 150mm range usually need 20 to 45 minutes. Anything larger can take 45 minutes to two hours. These are starting points, not fixed numbers: your specific acetone purity, ambient temperature, and container size all shift the timing.
Check the part regularly rather than walking away and coming back at the estimated time. Every few minutes, lift the lid briefly and look at the surface. You are watching for a uniform glossy, slightly wet-looking sheen across the whole part. The moment fine details start to look soft or rounded rather than crisp, the process has gone far enough and further exposure will blur them. Stopping a little early and doing a short second round later is much safer than overshooting and losing detail you cannot get back.
Some people speed the process up by placing the sealed chamber on a heated bed set to roughly 40 to 60°C rather than working at room temperature. This roughly halves the time needed, but raises the fire and vapor-pressure risk, so it is not the place to start if you have never done this before. Never try to heat the acetone itself inside a fully sealed container: that combination of confined vapor and a heat source is how you get an explosion, not just a faster smoothing job.
Safety equipment that actually matters
Acetone fumes are not a "crack a window" situation. Work outdoors or in a garage with the door fully open whenever you can, and aim for strong, constant airflow through the space, not just a fan pointed at your face. A basic dust mask does nothing against solvent vapor; you need an organic vapor respirator with the right cartridges. Pair it with nitrile or neoprene gloves and chemical splash goggles, not ordinary safety glasses, since acetone vapor and splashes both irritate eyes.
Acetone’s flash point is around minus 20°C, which means it can ignite at room temperature from a stray spark, a pilot light, or a nearby space heater. Clear any ignition source out of the room before you start, and keep a Class B fire extinguisher within reach if you are doing this regularly. Use acetone that is 99% pure or better. Nail polish remover often contains oils, dyes, and other additives that change how it reacts with the plastic and leave residue on the finished part.
Curing and handling afterward
The part is not done the moment you lift it out of the chamber. For the first 15 to 30 minutes the surface is still firm but tacky, and touching it will leave a fingerprint. By the 2 to 4 hour mark it is generally safe to pick up and handle without marking it. Give it 12 to 24 hours before treating it like a normal finished part, and be aware that the plastic’s full mechanical strength does not really come back for a few weeks, since the surface layer needs time to fully re-harden after being temporarily liquefied. Let parts cure somewhere well ventilated rather than in a sealed box, since they keep off-gassing a small amount of acetone as they dry.
Where this goes wrong
Warping shows up most often on parts with thin walls, since the acetone vapor softens the plastic on both the outside and, if the wall is thin enough, partway through the section, letting internal stress pull it out of shape as it cures. Printing with at least 2mm of wall thickness for anything going through this process gives you a much better safety margin.
Uneven smoothing, where one side of the part looks glossy and the other still shows layer lines, almost always comes down to vapor circulation. Rotating the part halfway through, or adding a small low-speed fan inside the chamber to keep the vapor moving, fixes most of it.
Over-smoothing is the mistake that is hardest to reverse. Fine text, thin ribs, and sub-millimeter details blur fast once the surface starts flowing, and there is no way to get that detail back once it has melted away. This is the main reason to check the part every few minutes instead of setting a timer and walking away.
Acetone vapor smoothing is one option among several in the broader post-processing toolkit, and it usually replaces sanding and filler primer for ABS parts rather than being used alongside them.