Hm, I cannot remember if buildings take DoT from liquid fire, I'm going to assume no...
However you could give the units you want to do the liquid fire attack a dummy ability that has it so when this unit with this ability attacks a structure it creates a dummy unit with an aura that only affects buildings but adds no bonuses to them, (ALTHOUGH, if you want easy DoT you could try unholy aura with negative values) Just check to see if there is already a dummy on this building to filter out excess dummy creation, set the expiration of this dummy to the duration you'd like the buff to last. As for the special effects you could either try and do something like moving the aura "buff" special effect overhead but I'm pretty sure it will look bad, alternatively you could create a special effect at the position of the attacked unit and destroy it when its finished.
As for the stopping repair, check when the unit is issued the repair order and if the building has the buff throw the error message and tell repairing unit to Stop
I hope this helps!