I would not apply any liquid but use a can of compressed air and blow across not directly perpendicular. There is a very thin film that usually is a filter and protectant that you do not want to possibly deform or dent.
Cleaning cd lens ?
Is it wise to open the unit and clean the lens? I have a Marantz DV9500 that will not play all cds. I suspect lens is dirty. Besides a q tip what should I use for liquid ?