I am looking for an anti magnetic spray for preventing magnetic particle ingress through a mesh cover?

Answer 1

No such thing as an anti-magnetic spray. If the mesh is a magnetisable steel, you could magnetise it to stop the particles passing. Otherwise line the mesh with a foam, gauze or porous paper filter to stop ALL particles of a harmful size, magnetic or not.

Answer 2

Your question is a bit hard to understand. I'll just answer it in two parts:

You want a paint that is not magnetic: Easy. Just find one that has no metal in it, especially iron.

You want a spray that prevents the underlying metal from being magnetic...
Well, that's not possible. Magnets work through anything.