smears, smudges on focusing screen

[jaehoppa]

Upon very close inspection of my focusing screen I've noticed some very faint cleaning marks and some finger prints. Are smear/smudging on a focusing screen permanent or can they be cleaned (professionally of course)? I'm not talking about scratches but smears and smudging. I've tested on a cheap focusing screen by "very" gently rubbing it with a folded corner of a microfiber cloth and it still created a faint mark on the surface. Is this permanent or reversible?
Thanks in advance.

 

[shutterfinger]

Focusing screens are fairly delicate and the general rule of thumb is to replace them.
I have had some success cleaning 35mm focusing screens, 2x3 and larger ground glass and fresnels under running water and Dawn dish detergent. Rub the ribbed side of fresnels and focus screens with the groves only. I give a final rinse in weak photoflo solution then stand on edge and allow to air dry. This procedure will result in very fine scratches on plastics. Others have cautioned me that some focusing screens are sandwiched and water will seep in between the layers.
Use your finger tips only to rub the screen. Apply pressure equivalent of rubbing an over ripe grape without moving/damaging the skin of the grape. Handle by the edges only.

 

[anfenglin]

If the screen can be removed as one part, maybe ultrasonic cleaning can help. I've never tried it but I know a guy who is an optometrist, he says it would work if it really is just one piece. Having something glued to it will ruin the glue.