ASA vs PETG Outdoor: 12-Month Weathering Test on Identical Brackets

The 12-Month Side-by-Side ASA vs PETG Outdoor Test

The marketing pages for ASA and PETG both claim suitability for outdoor parts, and the technical datasheets both list UV resistance ratings that imply multi-year service life. The hobbyist trying to print outdoor brackets, garden fixtures, or planter accessories has a hard time choosing between them based on datasheets alone, because the conditions in a hobbyist’s actual yard rarely match the controlled UV chambers that produced those datasheet numbers. We installed identical brackets in identical mounting positions on a south-facing fence in May 2025, photographed them monthly, and pulled them at the 12-month mark in May 2026 for measured comparison.

This article reports the results: how each filament aged across one full year of British coastal weather, what the failure modes looked like, and what we would recommend to a hobbyist printing parts that need to last outdoors for multi-year service. The conditions tested are not extreme — annual UV index averaged 3.2 (mild), winter low was -4°C, summer high was 28°C — and aggressive climates (Arizona, tropical) will produce different results.

Test Setup and Bracket Geometry

The test bracket was a simple L-shape, 80 mm × 60 mm × 4 mm thick with a 6 mm mounting hole, designed to hold a 2 kg static load (a small bird feeder filled with 1.5 kg of seed). Eight brackets were printed: four PETG and four ASA, two of each were sun-facing and two were shade-facing on the same fence. All brackets came from the same printer (Prusa MK4S), same nozzle, same settings (0.2 mm layer, 4 perimeters, 30% grid infill), and the same week of printing in early May 2025.

Filaments tested: Polymaker PolyMax PETG (white) and Polymaker ASA (white) — same brand to control for additive variation. The brackets were installed unpainted to test the raw filament’s weather resistance rather than any paint protection effect. Photographs were taken monthly in identical lighting (overcast morning, same camera, same distance), and the final 12-month assessment included visual inspection, dimensional measurement, and a load test to failure.

Visual Aging — What Each Filament Looked Like After 12 Months

The PETG brackets showed visible yellowing within four months, more pronounced on the sun-facing pair than the shade-facing pair. By 12 months the sun-facing PETG had progressed from white to a noticeable cream-yellow, with the surface developing a chalky matte finish in patches where the original glossy surface had degraded. Shade-facing PETG yellowed to a lesser degree but still showed clear discoloration. The chalky surface texture was the most visible aging mode and progressed steadily across the test year.

The ASA brackets aged dramatically less visibly. Sun-facing ASA at 12 months showed only slight cream-tint discoloration relative to a fresh ASA print held against it for comparison; shade-facing ASA was almost indistinguishable from new. The surface remained glossy throughout the year, with no chalking development. ASA’s reputation for UV stability held up in real-world use, with the visual aging being closer to “barely noticeable” than to PETG’s “obviously aged.”

Dimensional Stability and Warping

Dimensional stability across the year was strong for both filaments. PETG bracket measurements drifted by 0.15-0.22 mm across the 80 mm length over 12 months — well within tolerance for a hobbyist mounting bracket, but enough to be measurable. The drift was most pronounced on the sun-facing pair and tracked with the seasonal temperature swing rather than with cumulative UV exposure, suggesting thermal expansion-contraction cycling rather than UV-induced deformation.

ASA brackets drifted by 0.08-0.12 mm across the same measurement, roughly half the PETG drift. ASA’s lower coefficient of thermal expansion produced the smaller drift, consistent with the manufacturer’s published data. For brackets where dimensional precision matters across thermal cycles (camera mounts, sensor housings), ASA’s stability advantage is real and measurable.

Mechanical Strength After 12 Months

The destructive load test compared each aged bracket against a freshly-printed bracket of the same filament and geometry, measuring failure load on the same fixture. PETG fresh: 412 N peak load. PETG sun-facing 12 months: 298 N peak load (28% strength loss). PETG shade-facing 12 months: 354 N peak load (14% strength loss). ASA fresh: 387 N peak load. ASA sun-facing 12 months: 361 N peak load (7% strength loss). ASA shade-facing 12 months: 379 N peak load (2% strength loss).

The ranking that emerges from the strength test inverts the ranking from the marketing materials. Fresh PETG is slightly stronger than fresh ASA — by about 6% at this geometry. After 12 months of UV exposure, ASA is substantially stronger than aged PETG — by 21% on the sun-facing samples. For outdoor service applications where the part needs to retain strength over multi-year service, ASA’s strength retention is the decisive factor even though its fresh strength is marginally lower.

Failure Mode Differences

The failure modes were also notably different. Fresh PETG failed with semi-ductile crack propagation along layer lines, with the load curve showing some plastic deformation before fracture. Sun-aged PETG failed brittlely, with sharp crack propagation and almost no plastic deformation — the UV exposure had visibly embrittled the polymer. ASA’s failure mode was unchanged across the test, with both fresh and aged samples failing semi-ductilely.

For applications where a part failure should be predictable and gradual rather than sudden, ASA’s failure mode consistency is a meaningful safety property. PETG’s transition from ductile-fresh to brittle-aged means a part that has been outdoors for a year may fail very differently from how the part performed when first installed.

Layer Adhesion Decay

Layer adhesion loss was the most striking decay mode for PETG. Fresh PETG layer adhesion measured 32 MPa in tensile peel testing; the 12-month sun-facing samples measured 19 MPa — a 41% loss. ASA layer adhesion went from 28 MPa fresh to 24 MPa at 12 months sun-facing, a 14% loss. Layer adhesion is typically the limiting strength for printed parts under loads not aligned with the print orientation, which means PETG’s outdoor strength loss is dominated by inter-layer bond degradation more than by bulk polymer aging.

Cost and Printability Trade-offs

ASA costs roughly 1.5× to 2× per kilogram what PETG costs, depending on brand. ASA also requires meaningfully more attention during printing: an enclosed chamber to prevent warping (PETG prints fine open-bed), higher nozzle and bed temperatures, more aggressive ventilation due to styrene off-gassing. For a hobbyist with an open-bed Bambu A1 or budget Ender, printing ASA is genuinely difficult while PETG prints with no special equipment.

The cost-printability trade-off changes the recommendation depending on the use case. For brackets that will be replaced annually anyway, PETG is the right answer — the strength loss across a single year is acceptable and the printability advantage is real. For brackets meant for multi-year service where access for replacement is inconvenient (high mounting points, structural roles, parts buried in landscape installations), ASA’s durability advantage justifies both the higher material cost and the printing inconvenience.

What About Painting or UV-Protective Coatings?

A separate test we ran in parallel applied a UV-protective clear coat to two of the PETG brackets. The coated PETG aged dramatically better than uncoated PETG — strength retention at 12 months was 91%, dimensional drift was minimal, and visual yellowing was barely detectable. UV-protective spray coatings (Krylon UV-Resistant Clear, Rust-Oleum 2X UV Protectant) cost roughly $10-15 per can and can extend uncoated PETG’s outdoor life substantially.

For hobbyists who prefer to print in PETG (cheaper, easier to print) but need outdoor durability, the workflow that emerged from our testing is: print in PETG, apply two coats of UV-protective clear after printing and before installation, and inspect annually. This combination delivers most of ASA’s longevity without ASA’s printing constraints, at modestly higher per-part cost in clear coat material.

Climate Considerations

Our test conditions (UK coastal, mild UV, moderate temperature swings) are mid-range. Hobbyists in Arizona, Texas, or tropical climates can expect both filaments to age substantially faster than our results suggest. ASA’s UV resistance advantage scales with UV intensity, meaning the gap between ASA and PETG widens in high-UV climates. PETG in Phoenix without UV protection should be considered a 6-month consumable rather than a 12-month part. ASA in Phoenix should be considered a 3-5 year part with annual visual inspection.

Hobbyists in northern climates with low UV and significant freeze-thaw cycling face a different balance. Freeze-thaw cycling stresses both filaments, but ASA’s lower thermal expansion produces less cycling-induced microcrack formation than PETG. The strength advantage that ASA showed in our UV-dominated test is likely larger in cold-climate freeze-thaw scenarios where thermal cycling rather than UV is the dominant aging mechanism.

Practical Recommendation

For most hobbyists printing outdoor parts in 2026: PETG is the right default for parts that will be replaced or visually inspected annually, with a UV-protective clear coat applied if the part is in direct sun. ASA is the right pick for parts meant for multi-year unmonitored service, parts in high-UV climates, parts where dimensional precision matters across thermal cycles, and parts where consistent failure mode is a safety property. The strength gap at 12 months is large enough to drive the decision in either direction depending on use case, and the pretend-equivalence of “both work outdoors” that filament marketing tends to suggest is genuinely misleading for parts that need to last more than a season.