Amen, can't beat a good electric die grinder - I scored a super nice Metabo Pro model awhile back off sleazebay very cheap since someone had put a generator plug on it and the pawn shop "couldn't test it" , lol...$60 for a nearly new $380 tool. Very smooth, amazing torque and control versus even my low speed high-torque air type, love that thing.
Band files are tricky - it really is about getting the correct belt and work angle for what you're doing . Be careful, they can easily gouge the surface too far.
Flap disks - most people (no offense) never use them properly. They are not designed for putting heavy down pressure on them - if it's not cutting fast enough you need a more aggressive disk that doesn't load up nor burn off the grit .Try the CGW ceramic wheel type - they aren't cheap but will perform 10x better and last far longer and put far less heat into the metal. Do not put a lot of weight down on any flap disk, let the tool do the work or you risk warpage and they will not blend a surface flat, but create dips and valleys on each side of the weld. The weldment is harder metal than what's around it - light pressure will focus the cutting action on the weld, not force it to flex and cut on either side of it. CGW (Camel Grinding Wheels) also has some very good 1/8 cut/grind/blend wheels that work wonders - so does Weiler ,both about the same price range. Both companies also make some interesting flex grind hard wheels that are designed to blend welds directly and they do work pretty well - but again, far less down pressure and let the tool do the work.
A buddy of mine is an industrial sales guy for a big cutting tool company - so I get to test out a lot of the new products far ahead of them hitting the market - some very interesting stuff coming down the pipe this year..keep your eyes open.
Sarge