I can't pencil mod mine either. I have applied and reapplied the mod to the top of the pictured resistor and It did nothing.

Could they have sprayed like lacquer something over that area to prevent mods?

I can tell somethings different though when I hear you guys say you layed down a couple of strokes and changed the resistance dramatically.

Some one recommended sanding the top of the resistor could that work? I tried the other point aswell and neither gained me anything.

What dissolves lacquer? How about pure acetone hmm. I'll post back in a while.