difficult spots of dirt , DO you mean mold you could try a enzyme cleaner
Or a Knosti/spinclean & use it a a pre-wash before cleaning with fluid on the RCM & then rinsing with distilled water should remove all marks
Blurb from Mofi
Most sound degrading contaminants found on records are organic in nature, oils and sweat from hands,.skin. hair and other minuscule particles of dust are attracted by electrostatic charges on the vinyl and can become deeply embedded in the vinyl. The best way to remove these is with an enzyme specifically designed for this purpose. By coating the play surface with PLUS Enzyme Cleaning Fluid and allowing it to sit on the record surface for 3-5 minutes, the enzymes will break the particles down and loosen them from the grooves.
Mofi & Nagoaka best inners & used them for decades 50p to protect a £25-50 records is good value