To the best of my knowledge conformal coating requires very clean surfaces for proper adhesion (see doi: 10.1088/1741-2560/10/3/031002 ). At least in our lab all boards are cleaned using a complex cleaning procedure which has a rinsing with deionized water including conductivity monitoring as the final step (doi: 10.1109/EMBC.2016.7591102 ). For 100V a track clearance of at least 0.2 mm is recommended according to my information so 12 mil = 0.3048 mm should be fine ( http://www.creepage.com/ ). From my experience even the "no clean" (I typically use SMD291 and SMD291AX) can leave some serious amount of residue depending on the used amoung which can impact electrical signal quality. At least I observed signal degeneration in HF applications with 0.4 mm BGA ICs. After additional cleaning steps with deflux etc the signals typically normalize.

residuals after using SMD291 and only sloppy mechanical cleaning:

To the best of my knowledge conformal coating requires very clean surfaces for proper adhesion (see doi: 10.1088/1741-2560/10/3/031002 ). At least in our lab all boards are cleaned using a complex cleaning procedure which has a final "rinsing with deionized water including conductivity monitoring" step (doi: 10.1109/EMBC.2016.7591102 ). For 100V a track clearance of at least 0.2 mm is recommended according to my information so 12 mil = 0.3048 mm should be fine ( http://www.creepage.com/ ). From my experience even the "no clean" (I typically use SMD291 and SMD291AX) can leave some serious amount of residue depending on the used amount which can impact electrical signal quality. At least I observed signal degeneration in HF applications with 0.4 mm BGA ICs. After additional cleaning steps with deflux etc the signals typically normalize.

residuals after using SMD291 and only sloppy mechanical cleaning: