In order to properly polish it is best for the parts to be annealed (If they were originally case hardened). If not, it will be nearly impossible to be polished and the engraver cannot use normal tools. And if it is going to be case hardened in the end, it should be annealed anyway as part of the process. More modern gun alloys do not require case hardening and can be polished and engraved without annealing (such as Parker Repros for example).
The amount of engraving that is lost due to polishing is wholly dependent on if there is any damage or pitting on the surface that needs to come out. In many cases the issues needing to be worked out are just as deep as the engraving or even deeper. So the engraving must be wiped out in the prepping process.
The original engraving should be documented before any prep is done. Good photographs and maybe even ink/smoke pulls of the engraving are important. That way if anything is lost in prepping, the engraver has a reference to work from.
Normally when I have Gournet do recuts, I document with good photographs and I send them to him by email or printed with the parts and he always gets it right. Sometimes areas of the engraving get completely wiped out where only trace outlines are visible.
Again, the amount removed depends on how much needs to come off to give a good smooth defect free surface. Of course if it is very bad, micro welding can also be utilized to all the defects instead of taking down the surrounding metal.
I personally find that taking the metal to 320 or 400g is acceptable. And carding it with a stainless wheel with oil to blend the scratch marks. Obviously the higher graded gun, the more finely the surfaces should be finished to. But the coverage of the engraving can compensate for that too. When looking at original lower grade guns, it is important to note how many defects were actually left in the surface at the factory. But that is looking at it from a restorative standpoint.
Of course, some engravers may want to do all of the polishing and prep themselves, and they may remove material and recut as they go. But I would bet that most would prefer a ready-to-go canvas if they had a choice.
The ideal situation is if there is no damage to the metal and all you need to do is hit it with some 320 and 400 to shine it up and freshen the surface. But, most guns needing a recut have more damage than that and always need more work then you may think.
See here photos of a Parker CHE that I restored 4-5 years ago showing before and after prep as well as the after photos when the gun came back from Geoffroy by working from photos that I sent him.
Again, this is just what I personally do. It does not mean it is right.
Before 4.jpg
Trigger Plate 1.jpg
Polished 3.jpg
IMG_8875.jpg