trouble using bondo

Agree. Just finished buddy's 69 Charger body. weld in new metal, used evercoat/rage, then it was the pits to smooth it(and, no, it was not too high, as I used a 2' stick to check from good ends). It was crap sandpaper. Use 3m. Cuts good, no clogging. (We used 17" long air and hand files. And round tubing for some round contours; charger at the rear sail panel has too much rounded areas). Don't sand straight, go in a 45 degree x pattern from contour to contour. Then spread rage icing (it is so much easier to sand, and it fills in pits and scratches. Use straight edges on the lines, tape the lines to run filler to; then pull tape, run filler to what you just did.
We started with 36, then kept going up, finishing with 180. Then shot Sherwin Williams FP410 on it (2 part primer/heavy filler). then totally screwed it up. We sanded that with 180 NO! we sanded off the filler, but low spots really showed up. Lesson learned; Primering my 70 Duster at the same time; shot primer, used 400 grit on blocks(get the self adhesive stuff). Light pressure, just knock of the high spots- real low, more icing, kinda low, can't feel, but can see it, more paint filler.
And you don't have enough light, unless you pull the car out into the sunlight. And use undercoat or anything, to seal the weld on back of panel.
Also, we learned that without at least 1/8" gap, even stitch/ moving all over with a wet rag welding- A weld contracts while cooling, not enough gap, it expands outward (no good). 1/8" gap, it pulls inward (good).
The quality material costs more, but it is the only way you can get the repair right.