does the filter when look under the light appear like uniformly jit-liap jit-liap debris stuck onto the glass?
tink if it doesn't affect the picture, dun bother to clean it.
i tink the filter maybe permanently ruined after washing it with water, as it can seep thru the gap sandwiched between the 2 glasses.
tried on my old CPL filter and now even after years, there's ugly newton's rings in the glass interface....