[amptramp]

Excessive dirt, corrosion or stuff like tar from a smoker are bad for performance of a tuning capacitor.  There are small make-up applicators that are suited to applying cleaning chemicals in narrow spaces.  Even if you don't get it all off, it will clear dust bridging the rotor and stator plates and that may get rid of scratchiness during tuning.  Alcohol or various types of contact cleaner would work here.

After this, you clean off the bearings and lubricate them and then you clean the shorting fingers that ride in the groove in the tuning shaft.

Make sure you have contact across the straps that ground the shield sections between the capacitor sections.

Be sure the fibre pieces that hold the stator are free of dust.