Looks like you are close. I haven't worked with ABS but would guess you need to add more material and then sand it smooth. If the valleys are lower than you want the finished surface then you must add more material.
Body filler is very sand able and forms very hard. I would just add way more than you need then use coarse sand paper or a dremel to get it close then use increasingly more fine grit until it is very smooth. I would think 400+ is needed for final sanding.