A question popped up in a recent game. Do persistent damage effects (e.g. burn and corrode) from weapons that are dealing nonlethal damage (due to weapon fusions) deal lethal or nonlethal damage?
Burn and Corrode, specifically, deal their own damage completely independently of what delivered them - for example, you can use fusions to make a laser deal only cold and sonic damage, and the Burn will be type fire. This is because they are conditions. You'd have to use a fusion capable of modifying the delivered condition to change it in any way.
The Merciful fusion has no interactions with the Burn or Corrode conditions, and both will behave the same way with or without the fusion, i.e. they will deal lethal damage.