Band-pass corrected M31 data from WSRT RT9 × RTA with a strong HI signal. Top-left image: input data. The bright emission around 1420 MHz is from HI and should not be flagged. The vertical lines are instrument or RFI artefacts that should be flagged. Top-right image: after RFI detection without HI modifications, showing in pink what is flagged. Bottom-left image: after RFI detection using Stokes Q, U and V. Bottom-right image: after RFI detection using a specialized strategy for 1418–1424 MHz.