I am using this Power Metal Film Resistor - which as per the datasheet, is coated with a "red, non-flammable lacquer which provides electrical, mechanical and climatic protection". Now, on my PCB, I have two vias to GND placed underneath the power resistor placed for thermal relief in case the resistor gets heated up. Maybe that was my mistake, but the interesting thing is, part goes in to the field and is returned because smoke starts emitting from the resistor. On debug, I realize that the lacquer coating of the resistor had been chipped a little exposing the metal body of the resistor. There was very little clearance b/w the resistor and the PCB and in this particular case, the resistor happened to touch the via - boom! short to GND and resistor began heating up.
In this failing part, there might have been friction with another object or some mechanical stress that caused the coating to chip off, thereby exposing the potential problem (promixity to GND via). However, in other parts, assuming there is no friction - should I still expect the coating to come off, either due to normal ageing or due to moisture or thermal stress, etc?
It seems dangerous in general, if the coating of metal film resistors can come off due to mechanical stress - thereby exposing the metal body of the resistor to contact with say, the enclosure for example. Is this a known problem - how is this dealt with or prepared for?