Key takeaways
- UV destroys a part before heat or water do. Choose material for UV resistance, not water resistance.
- Outdoors, PLA turns brittle and chalky within months, even when it never gets wet.
- PETG resists water well but yellows and loses toughness in direct sun. It is a material for covered spots.
- Drainage and thick walls make up for more than most people expect — water trapped in a closed pocket is a common cause of failure.
The usual question is "which filament is most waterproof", and it is the wrong question. In a climate like Vietnam's, water is almost never what destroys a part first. UV does that, then surface heat, and only much later water.
Put those three in the right order and the choice of material becomes clear.
And here is the number underneath the whole article. According to the Ho Chi Minh City government portal, the city averages 27.3 °C over the year with an average annual relative humidity of 78.3 %, and about 1,800 mm of rain of which 93 % falls in the rainy season; only the dry-season months drop below 75 %. Hanoi is similar. For most of the year, both the spools in the workshop and the parts outside live where nylon and PETG keep absorbing water — see sources.
The first agent: UV
Ultraviolet radiation breaks polymer chains at the surface of a part. The result is not a softer part but the opposite: the surface turns brittle, fades and crazes with fine cracks, and those cracks become the starting point for mechanical failure.
| Material | UV failure mode | Outdoor use |
|---|---|---|
| PLA | Embrittles quickly, chalks, micro-cracks | No |
| PETG | Yellows, loses impact toughness | Covered spots only |
| ABS | Chalky fading, surface crazing | No |
| ASA | Very slowly — it is the grade designed for this | Yes |
| PC | Yellows gradually, keeps mechanical strength for a long time | Yes, with a coating |
| PA6-GF | Surface fibres up, reabsorbs moisture | Yes, if painted |
| TPU | Stiffens and loses elasticity | Short term only |
ASA is ABS with the UV-vulnerable component swapped out. The print settings are nearly identical, the price barely differs, and that is the whole difference between a part that lasts and a part that chalks after one season.
The second agent: surface heat
Outdoor air temperature is not the number that matters. A dark part in direct sun runs much hotter than the air around it, and if it is mounted on a metal roof or metal housing, it picks up conducted heat from that too.
- A light-coloured part in a breezy spot: usually still inside PETG's safe range.
- A dark part in direct sun: assume it exceeds 70 °C, which is outside PETG's range.
- A part on a metal roof or metal housing: add conducted heat, and only ASA, PC or PA6-GF make sense.
The third agent: water and humidity cycling
Water rarely attacks the polymer directly. It does damage in three indirect ways, and all three can be designed out:
- Water trapped in closed pockets. 3D prints have a lot of pockets. Water gets in and can't get out, sits there, and invites mould and corrosion of any metal fitted inside.
- Water between layers. A print with only two perimeters is not watertight even if the material is. Layer lines are leak paths.
- Humidity cycling in nylon. Polyamide absorbs water from the air, and at rainy-season humidity — on top of Ho Chi Minh City's 78.3 % annual average — it absorbs enough to soften and change dimensions. Outdoor nylon parts need a sealing paint.
Four design decisions for outdoor parts
- 01
Give water a way out
Every pocket needs a drain hole at its lowest point in the real mounting orientation, at least 3 mm across so dust doesn't seal it. This is the cheapest and most effective change on the list.
- 02
Make walls thicker than you think you need
UV only attacks the outer layer. A 3 mm wall with four perimeters keeps a healthy core after the surface has degraded; a 1.2 mm wall doesn't. Wall thickness buys lifetime.
- 03
Print solid where it must seal
For parts that must be watertight, use a 0.6 mm nozzle, 100 % infill in the wall region, and concentric top and bottom layers so the extrusion runs along the sealing face instead of across it.
- 04
Coat it if the material isn't UV-stable
A UV-resistant acrylic topcoat does more for PETG or PC than any print setting. It does not turn PLA into an outdoor material, but it noticeably extends the life of PETG and PA6-GF.
Quick selection by use case
| Use | Choose | Notes |
|---|---|---|
| Sensor housing, outdoor electrical box | ASA | Light colour to reflect heat, with drain holes |
| Clamps, roof brackets | ASA or PC | Check wind load too, not just static load |
| Garden parts under a roof | PETG | Cheaper, easier to print, no enclosure needed |
| Signs, decorative parts | ASA | Holds colour — its main strength |
| Structural load-bearing parts | PC or PA6-GF | Add a UV-resistant coating |
| Outdoor gaskets, pads | TPU 95A | Treat as a consumable; replace periodically |
| Irrigation, submerged parts | PETG or PC | Print solid, 0.6 mm nozzle |
A note on "biodegradable" PLA
PLA is called biodegradable, which leads many people to think it will dissolve outdoors. In practical terms the opposite is true: PLA needs industrial composting — controlled temperature and humidity with the right microbes — to break down. In nature it doesn't dissolve; it just turns brittle, fades and breaks into small fragments.
In other words, PLA is both a poor outdoor material and does not degrade outdoors. Use it for what it's good at: prototypes and indoor parts. The temperature windows for each material are in PLA, PETG or ABS, and the ASA category has the full print settings.
Frequently asked
Which 3D printing filament can be used outdoors?
ASA is the right choice for long-term sun exposure, because it has ABS's heat resistance without ABS's UV damage. PETG is acceptable under a roof. PC and PA6-GF work with a UV-resistant coating. PLA and ABS are unsuitable outdoors.
Is PETG UV resistant?
Not really. PETG resists water and mild chemicals very well, but in direct sun it yellows and loses impact toughness. It suits covered locations, short-term outdoor parts, or parts with a UV-resistant coating.
Are 3D printed parts waterproof?
The material is, but a printed part isn't automatically. Layer lines are leak paths. For a watertight part, use a 0.6 mm nozzle, print the wall region solid, use concentric top and bottom layers, and design a drain for every closed pocket.
Does PLA biodegrade if left outdoors?
No. PLA needs industrial composting with controlled temperature, humidity and microbes to break down. In nature it simply embrittles, fades and fragments, like other plastics.
How hot does a dark 3D printed part get in the sun?
Noticeably hotter than the surrounding air — a dark surface in direct sun often exceeds 70 °C in Vietnamese conditions, and more on a metal roof or housing. Measure with an infrared thermometer at the actual mounting spot rather than relying on the forecast.