dionnfr, yep, I agree with themightytimmah, replacement is the only way to go.
A couple of years back I sold my sports sedan to my nephew. It had a screen with a "safetygroove" (this is a groove which is ground into the lower edge of the screen just above where the wipers sit in home position. When the wiper moves across the groove it takes off an dirt, dust or plant material and makes the wipe much more effective and prolongs the life of the wiper rubbers).
To have the "safetygroove" done to a screen costs about A$450 and for this reason I explored getting the pitting and minor blemishes polished out of the 8 year old screen. There was no other option other than replace.