Fused filament fabrication (FFF) is one of the most researched techniques in the field of additive manufacturing because of its fast and cost-effective nature. However, the surface quality of these printed parts suffers because of the stairstepping effect, which leads to visible layer lines. Secondary finishing operations are often needed to comply with the design requirements. These post-processing techniques are useful but are labor-intensive and often time-consuming. The present study compares the effects of in-situ polishing of acrylonitrile butadiene styrene (ABS) using chemical and mechanical approaches. The advantages of in-situ settings include ease of operation and lower time consumption. For this, a modified rapid prototyping system was developed which had separate heads for mechanical and chemical polishing. A full factorial experiment was conducted with the two polishing methods and two feed rates. In results, mechanical polishing with end mill with a lower feed rate achieved the best roughness (Ra = 2.047 μm), while chemical polishing with a lower feed rate has the least improvement (Ra = 14.257 μm). General trends, pros, and cons for both chemical and mechanical methods are discussed.