HPL

ALWAYS TRY WATER AND MILD DETERGENT FIRST BEFORE TRYING THE FOLLOWING OPTIONS. SOMETIMES THE STUFF WIPES RIGHT OFF!

I would try regular car wax; which does act as a very minor "abrasive" and might allow you to simply buff the offending stain away with no further work required. Rubbing compound is the next step, but it will definitely take off a little paint; rebuff with finish wax to bring back to a shine if enough paint still exists. On older bikes (without clear coat) I will use a very weak solution of rubbing alcohol to remove various marks/stains, etc. I always test an area unseen to ensure I'm not just turning the paint into solution again. Start with max of 25% alcohol or lower (I use about 10-15% to start); you can always up the strength, but check again to make sure you're not taking off paint during usage. Soft pencil (definitely not ink!) erasers have also been helpful; just polish up with wax after and you can't tell after treating. Many clear coats are readily softened with alcohol if too strong; I avoid this unless as a final option. The clear coat can still be buffed with wax, but I tend to use "NicSand"; 3000, 5000, and 10,000 grit polishing pastes (10,000 is what I use). That stuff polishes anything (glass, paint, plastic), but you must be patient due to the extremely fine grit used to bring out a super shine without other waxes, polishing compounds, etc. I've used "NicSand" on car finishes to get a mirror like shine on paint and/or clear coat; but I put hours into the job to achieve the required result. I do not use paint thinners/solvents other than the alcohol (de-natured alcohol also works; diluted!). Hope this helps; I'm sure there are many other means of cleaning depending on the condition and/or requirements pertaining to a particular job. Just a note: I've had a container of "NicSand" for decades; it hasn't gone bad yet, just add a little water and it's good.