**2. Experimental Methods and Materials**

A Raise3D Pro2 dual extruder 3D printer (Raise3D, Irvine, CA, USA) was employed to produce samples from eSUN ABS and FilamentOne ASA PRO SELECT filaments using the FDM method. Three-dimensionally printed specimens had dimensions according to the ASTM D790 standard of 127 mm (length) × 12.7 mm (width) × 3.2 mm (thickness). Nozzles with a size of 0.4 mm were used for 3D printing of specimens and nozzles' temperatures were established to be 205 ◦C for ABS and 220 ◦C for ASA extrusion. The bed temperature and the infill density were set to 100 ◦C and 100%, respectively. The bed temperature is the temperature of the platform where the object is 3D printed. The infill density determines the distance between adjacent deposited filaments within the shell of the printed structure and an infill density of 100% results in no air gaps between adjacent filaments. The shell is the outer wall of the printed object that outlines the desired shape of the printed structure and provides an anchor for the infill deposition. The print pattern was set to alternating angles (i.e., periodic −45◦/+45◦) of layer deposition with solid edges to create identical mechanical properties in longitudinal and transverse directions of specimens, as illustrated in Figure 1. The pattern was printed with an inner shell speed of 70 mm/s, outer shell speed of 25 mm/s, and infill speed of 80 mm/s. The layer height for the deposition of ABS and ASA filaments was set to 0.3 and 0.125 mm, respectively. The layer height is the thickness of the deposited filament per pass. In ASA-coated ABS specimens, ASA coating with a thickness of 0.25 mm that consists of 2 layers was deposited on both sides of specimens. One of the advantages of AM compared to traditional manufacturing techniques is the ability to combine dissimilar materials in one continuous process, which eliminates the requirement of using intermediate adhesives for bonding two distinctive materials. The thickness of ASA coating was taken into consideration during 3D printing in order to manufacture uncoated and ASA-coated ABS specimens with the same thickness.

**Figure 1.** Deposition of the (**a**) first and (**b**) second layers during 3D printing of specimens to create a pattern of alternating −45◦/+45◦ angles.

To simulate the effects of outdoor environmental conditions on 3D printed polymer structures, ABS samples with or without an ASA coating layer were aged in a controlled environmental chamber. Exposure to UV radiation and moisture was conducted by using a UV radiation/condensation (Q-Lab QUV/basic) accelerated weathering tester. In the environmental chamber, the specimens were exposed to UV radiation and moisture cycles. UV radiation was generated with 340 nm wavelength UVA lights set at 0.89 W/m<sup>2</sup> intensity and 60 ◦C chamber temperature. The condensation was produced by condensing vapor on specimens in the environmental chamber with 100% humidity and a temperature of 50 ◦C. To follow the ASTM G154 standard and create a synergistic exposure to both UV radiation and moisture, specimens were cyclically exposed to 8 h of UV radiation followed by 4 h condensation.

The LEO 1530 Scanning Electron Microscope (Zeiss, White Plains, NY, USA) and VHX 6000 Keyence Digital Microscope (Keyence, Itasca, IL, USA) were used to characterize the surface morphology of specimens during environmental exposure. A 5 nm gold layer was deposited on samples via a Quorum sputter coater before SEM analysis and scanning electron microscopy was carried out with the beam accelerating voltage of 5 kV. The resistance of ASA coating to separation from the ABS substrate before and after environmental aging was assessed according to ISO 2409 standard. The flexural properties of specimens during environmental exposure were evaluated using the three-point bending test carried out by using Mark-10 testing equipment after 0, 300, 600, and 1200 h of environmental aging. The span length of 51.2 mm (span to depth ratio of 16:1) and the crosshead rate of 1.37 mm/min were used to produce a strain rate of 0.01 mm/mm/min in specimens based on the ASTM D790 standard. Six specimens per each condition were used to ensure the reproducibility of results.
