Anyone ever drill out their spray gun tip?

I have a cheap Harbor Freight gun that I'd like to modify to use a 2.0 tip to spray sprayable body fillers. I've read about guys drilling them out and then "lapping" the needle so it seats and seals at the tip. There's a video on youtube of a guy talking about doing it and he goes into detail on drilling it out but then just casually mentions lapping the needle without explaining it at all. I can't seem to find any information online on how to go about doing this. Anyone here ever do anything like this or know how to go about doing it?