Most “bad STLs” are a cold bed and a dirty sheet.

On an Ender: wash the PEI with dish soap, not IPA. Set bed 60 for PLA, 80 for PETG. First layer 0.24 mm, 30 mm/s, z-offset until the line squishes but doesn’t scrape.

Bambu: don’t fight the preset. If it won’t stick, the sheet has oil on it. Soap. Dry. Run the calibration again.

Prusa: live-z is the whole game. If the skirt looks like a round bead, you’re too high. If it looks transparent and torn, you’re too low.

Fix that before you buy another roll or another file. A print-ready STL still fails on a dirty bed.

If the printer itself is the problem — layer shifts, firmware brick, heat creep — that’s a live repair, not a new model.