I have the best solution you can thank me later.
After trying different sandpapers I found out that pure alcohol (methylated spirit) cleans like a champ without the need of sandpaper. When the electrode(s) start to stick on the nickel during welding put some spirit on a kitchen roll and clean/rub the tip and you are ready to weld again.
This 0.2mm nickel was welded with 30J
Lipo is nano-tech 3s 5Ah 65-130C
Here some pics from tests with 0.2mm nickel on a blade
bottom side of the blade
after removing the nickel with pliers
The wood underneath the blades got burned spots from welding
0.3mm nickel is also no problem above 50J but everything gets hot quicker.
Trying to remove 0.3mm nickel (80-100J) from a blade with pliers, impossible task.
One important thing I noticed is the angle of the electrodes during welding. It’s best to hold them with about 30° angle. Once I did some welds and they were poor with 30J. Even with 40J it didn’t look good and I thought there’s something damaged. Then I realized I held them with too less angle (maybe 15° only). After correcting the angle and went down to 30J it was perfect again, the welding spots are much bigger.