Sometimes the dust on your slide is too large for VueScan’s Infrared clean feature, and sometimes the slides have a grayish Goop on them. Generally these build over time while they are stored and should be removed before scanning.
Step #1 Remove the larger dust particles with:
1: Bulb dust blower (just search for a “bulb dust blower” at your favorite camera store)
2: Or, use a Compressed Canned Air blower, a brand name is Dust Off. (Caveat: canned air sometimes has a tendency to blow out some of its propellant along with the compressed air, which eventually evaporates, be careful not to spray propellant on the slide which could damage the film’s emulsion)
3: Once you have removed the larger dust particles you can use a dust free Optical Microfiber cloth (like to ones that an optometrist includes with glasses)
Step #2 Remove the goop
If under a loupe you see a grayish goop on the slide you MUST use a Photographic Solution that does not contain ANY water. Your alternatives are:
A: Isopropyl alcohol (it is water free as long as you use it properly) Or
B: Purchase a brand name solution like PEC-12 2.
C: Try and purchase a Kodak film cleaner, HFE-7200 Film Cleaning Solution.
If you are meticulous or if you have to remove the slide from its mount to properly clean the slide get a box of microfiber gloves. A brand name is Matin Microfiber Gloves Dust Fingerprint Proof (White).