Why this coating exists in the first place
Sanding PLA is miserable. The material gums up abrasive paper instead of turning to dust, so you end up smearing plastic across the surface instead of removing it. Acetone vapor, the trick that works so well on ABS, does almost nothing to PLA and barely touches PETG either. So for years, anyone printing in PLA who wanted a smooth, professional-looking part was stuck choosing between hours of futile sanding or just living with visible layer lines.
XTC-3D is Smooth-On’s answer to that gap. It’s a two-part brush-on epoxy, not a vapor treatment and not a filler primer in a spray can. It doesn’t dissolve the print’s surface the way acetone does on ABS. Instead it flows into the grooves between layer lines and cures into a thin, hard, glass-like shell sitting on top of the part, a different approach than spray filler primer, which builds thickness through repeated coats instead of a single self-leveling layer. That distinction matters, because it changes how you use it and what results you should expect.
What XTC-3D actually is
XTC-3D is a two-component epoxy system: a resin (part A) and a hardener (part B). You measure both, mix them, and brush the result onto a printed part before it kicks off and cures. According to Smooth-On’s own technical bulletin, the mix ratio is 2A:1B by volume, or roughly 100A:42B by weight if you’re measuring on a scale instead of eyeballing volume. Mixing by weight is more accurate, and with a working time this short, accuracy matters.
Once mixed, the two parts react and generate heat as they cure. This isn’t a cosmetic detail. Smooth-On’s data sheet warns that a large, concentrated blob of mixed XTC-3D in a cup can get hot enough to melt the cup, burn skin, or ignite nearby combustible material. Mix only what you’ll use in one application, in a proper container (not glass, not foam), and don’t leave a leftover puddle sitting around while you go do something else.
Mixing and working time
You get about 10 minutes of working time once the two parts are combined, a bit longer, closer to 20 minutes, if you’ve already spread it into a thin film on the part. That’s not a lot of time on a complex model, which is why most people mix small batches and work in sections rather than trying to coat an entire large print in one go.
Tack-free cure lands around the two-hour mark at room temperature. Full cure takes roughly 3.5 to 4 hours depending on how thick the coat is and how warm the room is. If you’re in a hurry, mild heat (around 150°F/65°C) speeds that up dramatically, cutting cure time down to about 15 minutes, though most home setups aren’t doing controlled heat curing and room temperature is fine for a hobby project.
How to apply it
Grab a cheap chip brush or a foam brush. Foam gives you a thinner, more controlled coat, which is generally what you want on a first pass. Dip, then brush a thin layer over the part, working it into the layer lines rather than just laying resin on top. XTC-3D self-levels reasonably well and doesn’t leave visible brush strokes if you keep working the surface while it’s still fluid, which is the real skill here: keep the brush moving and keep redistributing material until the resin starts to set up and resists your brush. That’s your cue to stop.
This is the step most people rush. If you walk away and let the coat sit still while it’s curing, gravity does what gravity does and you get sagging, pooling in recessed details, and drips along the bottom edge. On a model mounted vertically, mounting it on a stick or dowel and periodically rotating it while the coat sets prevents a lot of this.
If a part needs more than one coat, and thin-walled or heavily textured prints often do, let the first layer cure to what’s usually described as "tacky hard" before brushing on the next. Applying a second coat over a fully cured, glassy-smooth first coat gives you weaker adhesion between layers.
Sand before, and sand again after
This is the step most people skip entirely, and it’s the reason some people finish a coat and still see every layer line staring back at them. A quick pass with fine grit, around 220, before you ever mix the epoxy knocks down the fuzz and whiskers that FDM prints are covered in and gives the epoxy something to mechanically key into. Skip this and the coating just follows the existing surface texture instead of masking it: you get a glossy version of the same rough part, not a smooth one.
After the epoxy has fully cured, another light pass with 220-grit wet sandpaper knocks the shine off and gives primer or paint something to grip. XTC-3D cures to a hard, glassy finish, and paint doesn’t adhere well to glass. If you’re planning to prime and paint afterward, this second sanding pass isn’t optional either.
What it works on
XTC-3D is compatible with a wide range of printed and non-printed materials: PLA, ABS, PETG, nylon, HIPS, and SLA/SLS resin prints, along with powder-printed parts. It also works on non-print materials like wood, plaster, cardboard, and rigid foam, which is why model makers and prop builders use it well outside the 3D printing world. PLA is where it earns its keep the most, since that’s the material acetone can’t touch and traditional sanding fights you on.
Safety: this is not the resin you used for jewelry
Two separate confusion points come up constantly with this product, and both are worth clearing up before you buy a kit.
First: XTC-3D is not acetone, and it’s not used the same way. There’s no vapor chamber, no sealed container full of fumes doing the work while the part sits untouched. You’re applying liquid directly with a brush, by hand, and it cures through a chemical reaction rather than evaporation.
Second: XTC-3D is not the same product as the clear casting resin sold for jewelry, tumblers, or river tables. Both are epoxies, but they’re formulated for different jobs. Jewelry resin is optimized for optical clarity and slow, low-exotherm curing in molds. XTC-3D is formulated as a thin, self-leveling surface coating with a much shorter working time. Using one in place of the other gives you a worse result at best.
On the safety side, Smooth-On states XTC-3D doesn’t contain VOCs, phthalates, or phosphates, but that doesn’t mean it’s harmless to work with. It’s irritating to skin and eyes, and the safety data sheet recommends working in a well-ventilated area with a NIOSH-approved respirator, plus gloves and long sleeves to avoid repeated skin contact. Treat it with the same care you’d give any two-part epoxy: gloves on, windows open, and don’t let it sit on bare skin.
Common mistakes worth avoiding
The biggest one is applying too much material at once. A thick, generous coat feels like it should give better coverage, but it’s the fastest way to get sagging and drip lines, especially on vertical surfaces or overhangs. Thin coats, applied in multiple passes if needed, beat one heavy pour every time.
The second is skipping the pre-coat sanding pass and expecting the epoxy alone to erase layer lines. It fills small gaps and gives a glossy surface, but it isn’t a gap-filling primer in the way automotive filler primer is. Heavy layer lines on a poorly tuned print will still show through a thin epoxy coat; they’ll just be glossy instead of matte.
The third is mixing in the cold. XTC-3D cures slower at low temperatures, and a cold, damp workspace can leave you with a tacky surface that never fully hardens or that pools before it sets because it’s staying liquid longer than expected. Room temperature, ideally above 70°F/21°C, is where the working time and cure time on the data sheet actually apply.
What the finished part looks and feels like
Done right, a part coated in XTC-3D looks noticeably different from a raw FDM print: glossy, dense, with layer lines either gone or heavily muted depending on how much sanding prep went in beforehand. The surface hardens to a Shore D rating around 80, similar in feel to a hard plastic rather than a soft coating, which is part of why it holds up to handling and light impact better than paint alone. It’s the closest a home FDM print gets to looking like it came out of an injection mold, and for props, RC parts, or anything that needs to survive being picked up and handled repeatedly, that hardness is doing real work, not just cosmetic work.
For the sanding technique that gets a print ready for its first coat, see our sanding guide. For the full sequence of post-processing steps beyond this one, see the post-processing overview.
A cured XTC-3D shell helps close up surface gaps, but it is not a substitute for the wall count and infill choices that actually determine whether a part holds water. See our guide to waterproofing 3D prints for the full picture.