Begun pre-prime etch scrub and thoughts about material loss

Some elevator ribs, after wet scrubbing with 2 parts demineralised water/1 part Stewart Aircraft Systems EkoEtch and Festool Vlies 320 grit pad. The wet parts were left for aprox 2 minutes then rinsed with tap water (very soft in Melbourne) then air dried.

A question - how much material is removed with all this scrubbing, from the initial part as delivered, to this stage? Some informal testing indicates about 5-10 microns. From communication with TAF arose a recommendation to not reduce thickness by more than 50 microns, in the context of a 1 mm thickness spar. The ribs tested here have nominal thickness 0.5mm and 0.64mm, so treating this recommendation proportionately, the amount of material lost is well under the limit (would be 25 and 32 microns respectively). I would like to reduce this number.

Going forward, the aim is to do less scrubbing, particularly in any steps prior to the etching. Any use of the 100/320 grit pads will be to smooth edges with an avoidance of surface rubbing as far as possible, until this etching step.