OK, I just realized something (by accident): the solvent in the Mig Ammo filter dissolves the Mig Ammo acrylic paint… Never had this issue with Tamiya paints I normally use, so it must be a Mig Ammo paint specific issue.
If you want to use this as an intentional chipping technique, the trick is to use a brush and gently rub the surface- in no time you will uncover the primer. Now this can be a bad thing, when you do not expect it (like I did not expect it…), but I chose to look at this as a happy little accident.
My Ferdinand is somewhat chipped… although the underlying color is dark grey -not the best for chipping.
Looking at the results I managed to get a rubbed-off, worn effect – not stark chips you would get using salt/hairspray/chipping fluid/sponge. I think I will put this to the other methods I have been experimenting with.