Stop reprinting the same fail: a 10-minute nozzle check
Most "bad STLs" are a half-clogged nozzle.
If the first layer looks chewed, walls look fuzzy, or underextrusion shows up after 20 minutes, do this before you reprint:
Heat the nozzle to filament temp + 10°C.
Pull the filament. Look at the tip. Skinny, burnt, or a blob — that's your clue.
Cold pull (nylon or cleaning filament if you have it; PLA works in a pinch): heat, push, cool to ~90°C, pull. Repeat until the pull comes out clean.
Check the PTFE / heat break seating. A 1mm gap looks exactly like a "bad file."
Re-slice one 20mm calibration cube at your normal profile. Cube fails → it's the printer. Cube is clean and the part still fails → then look at the STL (overhangs, wall thickness, unprintable gaps).
Hobby FDM (Bambu, Creality, Prusa) fails in the same three places: nozzle, bed, and wet filament. Files don't unclog a nozzle.
That's the 10-minute check. Do it once. Then reprint.