How do you seal a 3D print with epoxy?
Mix a two-part brush-on epoxy, brush one thin coat over the whole surface and let it cure. Smooth-On’s XTC-3D mixes 2:1 by volume, gives 10 minutes in the cup, is tack free in 2 hours at 23 °C and cured in about 4. Prusa Research’s leaking PETG box became fully watertight after one epoxy treatment.
The procedure
Figures are from the XTC-3D technical bulletin. Other brush-on epoxies differ, so read the sheet that came with yours.
- Work at room temperature, with protection. Smooth-On specifies 23 °C, a well-ventilated area, a respirator approved by NIOSH (the US National Institute for Occupational Safety and Health), gloves, long sleeves and safety glasses. The mix generates heat and can burn skin.
- Measure accurately. 2 parts A to 1 part B by volume, or 100:42 by weight. Use plastic, metal or wax-free paper cups, never foam or glass.
- Mix for at least one minute. Scrape the sides and bottom several times with a square-edged stick.
- Pour it into a flat foil tray. Pot life in a cup is 10 minutes; spread thin in a tray it is 15, and a thin film stays workable for 20.
- Brush one thin coat with a foam brush, under 0.4 mm. Keep working the surface until the resin starts to set so it does not run. Coat complicated prints one section at a time.
- Recoat if needed once the first layer is tacky hard, within about 90 minutes.
- Cure. Tack free in 2 hours at 23 °C, cured in a thin layer in 3.5 hours. Mild heat of 60 °C cures it in 15 minutes, but some print materials deform above 93 °C, so keep the oven low.
- Fill and test before trusting it. The leak test is a night on a dry tissue.
One 28 g mix covers 651 cm² at that thickness, so a small kit does several parts. The cured coat is Shore D 80, sandable and paintable. Smooth-On lists PLA (polylactic acid), ABS (acrylonitrile butadiene styrene) and PETG (glycol-modified PET) among the plastics it bonds to.
Which coating for which job
Prusa tried four ways to seal a print that still leaked after the slicer changes on the watertight guide:
| Coating class | How it goes on | Result in Prusa’s tests | Food contact |
|---|---|---|---|
| Two-part epoxy | brushed, cured hours to days | leaking PETG box became fully watertight; also improved mechanical properties | only a certified food-contact epoxy, cold use |
| Acrylic lacquer spray | sprayed inside and out a few times, left to dry | seals surface pores; Prusa gives no depth figure | Prusa advises against it for food |
| Chemical smoothing | acetone vapor, ABS, ASA, HIPS and PVB only | acetone-smoothed ASA still leaked slowly in the closed-box test | not addressed |
| Nail polish | brushed from the bottle | works on small areas; impractical and expensive | not addressed |
Epoxy is the only class Prusa found that both sealed a failed part and can be made food safe. The cost is fumes, gloves and, in Prusa’s words, a cure that may take several days for the resins it used.
Sealing PLA
Nothing in the procedure changes for PLA. Smooth-On names PLA as a supported surface, and Prusa’s part 1 lists lacquer and epoxy as options for every material. The one PLA-specific rule is the oven: PLA softens above 60 °C, so cure a PLA part at room temperature rather than the 60 °C fast cure.
Food-safe epoxy
A watertight coat is not a food-safe coat. Prusa’s food-grade study used Efkoresin Art UV, an epoxy certified under EU regulation 10/2011, and after 14 days of simulated use the coated print grew no bacterial colonies while the untreated print grew the most. Resin Pro’s Epoxyfood is another resin built from the 10/2011 positive list: 100:55 by weight, workable after about 10 hours, fully cured in 24 to 48 hours, and rated to 70 °C.
Three rules follow from those sources:
- Cured only. Prusa notes that every epoxy resin is toxic in liquid form. Food contact starts after the full cure the maker states, not when it feels dry.
- Cold use only. Prusa rules out chopping boards, hot soups, microwaves and dishwashers for coated prints. Resin Pro certifies its resin for cold snacks.
- Personal use. Prusa holds no food-contact certification for its filaments and says such prints are not for sale.
A general-purpose print coating like XTC-3D carries no food-contact statement in its data sheet, so treat it as a sealant for vases, housings and outdoor parts, not cups.
Sources
- XTC-3D High Performance 3D Print Coating, Technical Bulletin (Smooth-On) (accessed September 23, 2026)
- XTC-3D COAT Product Information (Smooth-On) (accessed September 23, 2026)
- Watertight 3D printing PT1: Vases, cups and other open models (Prusa Research blog) (accessed September 23, 2026)
- Watertight 3D printing part 2: Airtight closable models (Prusa Research blog) (accessed September 23, 2026)
- How to make food-grade 3D printed models (Prusa Research blog) (accessed September 23, 2026)
- Epoxyfood, certified food contact transparent epoxy resin for coating (Resin Pro) (accessed September 23, 2026)