Well, sometimes the disc needs a good scraping....  especially after some thick and rich fares...

You may see some buildup on the sides like on this 24" Rio Grande Disc - you can see a bit of build up on the uppermost portion of the disc...

So, get one of these  - -

Now use it to scape the side --- AND BE CAREFUL not to gouge the metal...

Here is what you may see when scraping....and just wipe with a paper towel...

After a nice scrape the disc is shiny and sleek...

(*Thumbnail courtesy of ZAK - The Magic Bus (Alaska)).