**1. Introduction**

Additive manufacturing (AM) has numerous applications, ranging from medicinal delivery to aerospace, automotive systems, construction, biodegradable solutions, and ever-expanding technologies as a platform for innovative designs [1–4]. Three-dimensional printing technology found its way into different industries through a variety of techniques such as powder bed fusion, inkjet printing, direct energy deposition, and laminated object manufacturing [5,6]. Given the expanding range of processes, the scope of solutions has likewise expanded, from the 3D printing of shape-memory materials in the development of aerospace deployable equipment in solar panels and antennas [3], and thermoplastic equipment that can be printed on board the international space station, to manufacturing of biodegradable scaffolds for organ growth, and direct printing of organs and tissues [7]. The process has been adopted as one of the industrial requirements for technology and scientific research. The most common method of 3D printing is fused deposition modeling (FDM). In this technique, a thermoplastic material, typically in the form of continuous filaments, is heated and then extruded through a nozzle into several layers to form the final object as it is being cooled down [8–13].

Polymer matrix 3D printed structures, like other types of polymer-based structure, are susceptible to environmental exposure, such as ultraviolet (UV) radiation, moisture, or heat. The diffusion of moisture into the structural components can damage the material as a result of the change in the mechanical properties of the polymer or replacing or weakening matrix-reinforcement interfacial bonds through irreversible hydrolysis and plasticization [14,15]. Ultraviolet photons from sunlight exposure initiate photo-oxidative reactions (i.e., chains scission and chain crosslinking) which deteriorate polymer materials by altering their chemical structures. Chain scission reduces the molecular weight of the polymer, which in turn degrades its strength and heat resistance. Chain cross-linking enhances the brittleness of the polymer, leading to the surface microcracking [16–19]. The synergistic environmental exposures can be more detrimental than individual exposures acting alone. For instance, in outdoor environments, moisture diffuses into polymer materials and occupies positions among the polymer molecules that results in the swelling of polymers. The moisture-induced damage will be exacerbated by microcracking formation during UV exposure, which facilitates the moisture diffusion into polymers. Moisture also dissolves and removes products of photo-oxidative reactions and exposes a fresh surface for further degradation by UV radiation [20–22]. The adverse effect of moisture increases at elevated temperatures as a result of an increase in the rate of water absorption into polymer-based structures [23,24].

Nowadays, 3D printers can create multi-material objects with desired properties in specific locations. By the advancement of AM processes, the creation of multifunctional parts becomes feasible, which has never been possible through traditional, single-material manufacturing methods. This unique AM feature is possible through layer-by-layer placement of material in the specific areas, which enables the manufacturer to control structural properties at exact locations and tailor them for specific applications [25–27]. Multi-material additive manufacturing can produce coatings with certain properties on the surface of 3D printed structures, and it is a great replacement for conventional ways of painting structures, such as brushing and air-spraying, or recently developed methods for metallization of structural surface that are more costly and time-consuming [28]. The compatibility between 3D printing materials as well as the printing parameters such as nozzles' temperatures and printing speed are important factors in creating a strong adhesion between 3D printed layers with distinct material properties using multi-material additive manufacturing methods [5,25].

ABS, or acrylonitrile-butadiene-styrene, has seen the largest commercial usage in the additive manufacturing industry due to its rigidity and high mechanical performance. ABS can be used to create objects with intricate designs, structures with moving parts, and structures that are aimed for further plastic forming, without the risk of breakage. However, it has been reported that the mechanical properties of 3D printed ABS structures can diminish significantly during exposure to harsh environmental conditions [28–30]. ASA, or acrylic-styrene-acrylonitrile, is a weather-resistant thermoplastic material with a high performance in outdoor environmental conditions, including UV radiation, moisture, and high temperatures. It offers easy printing, good dimensional stability, and an excellent layer to layer adhesion.

In this paper, the surfaces of ABS 3D printed structures were coated by weather-resistant ASA using a multi-material additive manufacturing technique to enhance the durability of the structure in aggressive environments. The current study offers insights for the design, coating, and maintenance of 3D printed polymer structures exposed to outdoor environmental conditions.
