But you're not going to change angle with a diamond stone right or do you with a 200 grit diamond for example (for example when speaking about rental skis with maybe a different base edge angle)?
You can change the angle on the base edge with a diamond stone. If this is your goal we recommend a 400 grit stone and finish with 600 or 1000 (if possible) but not mandatory.
But you're not going to change angle with a diamond stone right or do you with a 200 grit diamond for example (for example when speaking about rental skis with maybe a different base edge angle)?
You can change the angle on the base edge with a diamond stone. If this is your goal we recommend a 400 grit stone and finish with 600 or 1000 (if possible) but not mandatory.
Is 0° base edge angle on a fis slalom ski doable for a week of freeskiing on unforeseen different conditions (on piste) in your opinion?
No, you'll need a base bevel of your preference. Most likely 0.5 degrees on a SL ski.