How to make a 3D-printed vase watertight

FDM prints are built from lines of plastic laid side by side, and water finds its way through the tiny gaps between them. A vase that looks perfect can still weep. This note collects what works to close those gaps, from the slicer to the finish.

The principle

Leaks come from three places: gaps between neighbouring lines, poor fusion between layers, and the seam where each layer starts and ends. Every setting below attacks one of the three. The thicker and better fused the wall, the less it matters which filament you use.

Two ways to print a vase

Vase mode with wide lines

Vase mode (spiral mode, or “spiralize outer contour”) prints the wall as one continuous line climbing in a spiral. There is no seam, which removes the main leak path, but the wall is a single line thick, so it has to be a good one.

Widen the line. With a 0.4 mm nozzle, push the line width well above the nozzle, up to around 0.8 mm. A wider line makes a thicker, stiffer wall. A larger nozzle (0.6 or 0.8 mm) does this even better.
Print slower than usual, so each layer has time to fuse to the one below.
Print hotter, about 5–10 °C above your normal temperature.
Raise the flow slightly, a few percent, so the line presses into its neighbour.

Vase mode only works on simple shapes: no flat tops, no steep overhangs, no internal features.

Normal mode with thick walls

For complex shapes, print normally and make the wall thick. Tests by Prusa found that the number of perimeters is the single most important factor: aim for at least four, better 4–6, which gives a wall of about 2–3 mm. With walls that thick, the choice of filament matters little.

Layer height: 0.15 mm or lower. Thin layers fuse better than thick ones.
Bottom layers: at least 5, so the base does not leak.
Flow: 105–110% if you can accept a rougher surface and some stringing.

Material

PETG is the usual choice for vases: it resists water better than PLA, which can soften and turn cloudy if it holds water for a long time. PP (polypropylene) is the most watertight of the common filaments but harder to print. With thin walls the difference between materials is large; with thick walls it shrinks.

When settings are not enough: seal it

Clear acrylic varnish sprayed inside and out in a few coats: the simplest fix, but not food-safe.
Food-grade epoxy resin poured and rolled inside: effective and food-safe once cured, but messy and slow.
Melted wax or a thin layer of silicone on the inside: an old trick that works for flower vases.

A quick checklist

Setting	Vase mode	Normal mode
Walls	1, wide (≈ 2× nozzle)	4–6 perimeters
Layer height	Low to medium	0.15 mm or less
Temperature	+5–10 °C	+5–10 °C
Flow	+ a few %	105–110%
Speed	Slow	Normal to slow
Bottom layers	5 or more	5 or more

Test every new vase with water over a sheet of paper before you trust it on a table. And keep printed vases out of the dishwasher: the heat warps PLA and many PETGs.