Sealing 3D Prints: PTFE Tape, Epoxy and TPU Gaskets

A 3D-printed fitting can be dimensionally perfect and still leak. On a boat that matters: a slow weep behind a locker liner turns into a wet bilge, a smell, and eventually a corroded terminal block. The good news is that sealing a printed part is not mysterious — it is the same problem as sealing any plastic part, with one extra variable: the layer lines.

This guide covers the four sealing methods worth knowing, when each one is the right answer, and the mistakes that quietly ruin an otherwise good print.

First: understand where prints actually leak

FDM prints leak in two different places, and they need two different fixes.

• Through the wall. Water finds its way between layers or between the perimeters (the "wall" loops) of the print. This is a print-quality problem — the fix is in the slicer, not in a tube of sealant. We covered it in detail in How to 3D Print Watertight Boat Fittings That Hold Pressure.
• Across the joint. Water escapes at the interface — a threaded connection, a flange against a bulkhead, a cap against a rim. This is what sealants and gaskets are for, and it is what this article is about.

If you try to solve a through-the-wall leak with more PTFE tape, you will just chase it around the fitting forever. Print the part properly first, then seal the joint.

PTFE tape: the default for threaded joints

For a tapered thread (NPT, BSPT) carrying water at low pressure, PTFE tape is still the simplest and most reliable answer. It fills the helical leak path between the crests and roots of the thread and lets the joint tighten without galling.

How to use it on a printed thread:

• Wrap in the direction of tightening — looking at the male end with the thread facing away from you, wrap clockwise. Wrapped the wrong way, the tape unravels as you screw the parts together.
• Use 3 to 5 wraps, pulled snug enough to conform into the thread valleys. Printed threads have a slightly rougher surface than machined ones, so a little more tape is usually better than a little less.
• Start one thread back from the leading edge, so you do not push shreds of tape into the bore.
• Use the right density. Thin white plumber's tape is fine for small fittings; the thicker pink or yellow (gas-rated) tape gives more body on larger diameters.

The important caveat with printed plastic: tape is a wedge. Every wrap adds interference, and on a tapered thread that interference becomes hoop stress in the female part as you tighten. Plastic will split long before brass does. Snug plus a quarter turn is the target — if you find yourself reaching for a bigger wrench, stop and re-cut the joint instead.

For parallel threads (BSPP, and most of what we design for at MarineLab3D — see BSP vs NPT Threads for Boat Plumbing) tape alone is not the sealing mechanism. A parallel thread seals on a face: an O-ring, a bonded washer, or a flat gasket at the shoulder. Tape on a parallel thread does very little except take up slack.

Thread sealant (liquid PTFE / anaerobic): better on rough threads

Paste and liquid thread sealants flow into the surface texture of a printed thread far better than tape does, which makes them a strong choice when the thread finish is not perfect — exactly the situation with a 0.4 mm nozzle and a fine pitch.

• Non-hardening PTFE paste (the classic pipe dope) is our default recommendation. It seals, it lubricates during assembly, and the joint remains serviceable — you can take it apart in two seasons without destroying the part.
• Anaerobic thread lockers/sealants (the Loctite 5xx family) cure in the absence of air between metal surfaces. On plastic they cure slowly or not at all without an activator, so check the datasheet before you rely on one.
• Avoid aggressive solvent-based sealants on printed parts. Some contain agents that will craze or soften plastics. If a datasheet does not list your material as compatible, test on a scrap print first — 24 hours in a jar with a section of the same filament tells you a lot.

Epoxy and coatings: sealing the part itself

When you want a printed part to be impermeable — a tank fitting, a housing, anything that will sit in standing water — a thin epoxy coat closes the porosity that slicing leaves behind.

The method that works:

• Use a low-viscosity laminating epoxy, not a five-minute hardware-store epoxy. Low viscosity wicks into the surface instead of sitting on it.
• Warm the part slightly (30–40 °C) before you brush the epoxy on. As the part cools, air contracts and pulls the resin into the voids. Do not overheat it — PLA and even PETG will sag.
• Thin coats, two of them, sanded lightly between. A thick coat traps bubbles and cures glossy but porous underneath.
• Mask the threads and any mating surface before coating. Epoxy in a thread ruins the fit, and you cannot sand it out cleanly.

For parts living on deck, add UV to the equation: unfilled epoxy chalks in sunlight. Either topcoat it with a UV-stable paint or print in ASA in the first place. Material choice does most of the work here — see Best Materials for 3D Printing Boat Parts.

TPU gaskets: the underrated option

The most elegant fix for a face-sealed joint is to print the gasket. Flexible TPU (Shore 95A is the sweet spot for gaskets — soft enough to conform, stiff enough to print cleanly) makes an excellent, cheap, exactly-sized seal for flanges, hatches, cap rims and inspection ports where no off-the-shelf O-ring fits.

Rules that make printed gaskets work:

• Thickness 1.5–2.5 mm. Thinner than that and it prints poorly; thicker and it squeezes out sideways instead of compressing.
• Design for 15–25% compression. Below 10% it will not seal reliably; above 35% you are just extruding the gasket out of the joint.
• 100% infill, concentric or aligned top/bottom layers. Any infill pattern is a leak path straight through the gasket.
• Print it flat, on a smooth plate. A textured plate leaves a texture that will weep.
• Give the flange a shallow groove or lip to retain the gasket. Free-floating gaskets migrate.

TPU also handles vibration and thermal cycling better than a rigid joint, which is exactly what an engine bay or a hard-driven hull gives you.

Which one, when

Joint	Best method	Notes
Tapered thread (NPT/BSPT), low-pressure water	PTFE tape	3–5 wraps; do not over-tighten plastic
Printed thread, rough finish	PTFE paste	Fills texture; stays serviceable
Parallel thread (BSPP)	Bonded washer or O-ring at the face	Tape is not the seal here
Flange to bulkhead / hatch rim	Printed TPU gasket	1.5–2.5 mm, 100% infill, 15–25% compression
Tank or housing wall porosity	Low-viscosity epoxy, 2 thin coats	Mask threads; UV-coat if on deck
Permanent, non-serviceable joint	Solvent weld or structural adhesive	Match the adhesive to the polymer

Test before you trust it

Every sealed assembly deserves a bench test before it goes into a locker you cannot reach. Cap one end, fill the part with water, and leave it standing on a paper towel overnight — a slow weep will show up as a stain long before it shows up as a wet bilge. For anything pressurised, do the test at working pressure with the part inside a bucket, and hold it for a full ten minutes rather than ten seconds.

Most of our parametric fittings are designed with sealing in mind — proper thread profiles, flat sealing shoulders and generous wall thickness. Browse the Plumbing & Pumps collection for fittings, or the free files if you want to practise the technique on something that costs nothing but filament. If you need something specific, drop us a line at info@marinelab3d.com.

Safety note

3D-printed parts are not certified marine equipment. Sealing methods described here improve a joint but do not make a printed part suitable for critical applications. Do not use printed fittings below the waterline, in fuel or gas systems, in steering, or in any load-bearing or life-safety role without proper engineering assessment and testing. Always verify material compatibility with the fluid involved, and inspect printed parts regularly.