WD-40 and similar products are more solvents and cleaners than lubricants. They are thin to enable them to get into tiny crevices and contain penetrants to loosen up rusty parts.
However, WD-40 should not be used for rusty bearings, because even moderately rusted bearings are already damaged beyond repair.
Check out Cleaning Bearings to see what we recommend as a great cleaner.
WD40 is great for a variety of applications, but is not perfect for cleaning or lubricating skateboard bearings.
It is not the best for cleaning bearings because you still need to add a proper bearing lubricant after using WD-40. Since WD-40 degreases, it will not allow your lubricant to work as intended, because WD-40 works to remove it.
Don't get too excited over the appearance of bearings spinning fast after using WD-40. After a short period of time, the bearings may become damaged and not work at the intended optimum performance. The reason is the complete lack of a proper lubricant.
At Rat Vision, we don't use WD-40 or similar products for anything skateboard related.