I think you have an anodized steel drum in these, they do not rust. If they are not anodized and rusty, the only solution I know is to remove the rust spots with emory paper at the beginning of rental, daily use will prevent rust buildup.
If anodized, you probably have worn drum front glides, Pn AP3156144.

I have a Kenmore dryer that's leaving thin rust stains. We thought it was the washer but it wasn't. I discovered that the rust is coming from the felt washer between the rear bulkhead and the drum. It appears the clothes are getting pinched in the gap as they tumble. I looked up the parts but it would be pretty expensive to replace the parts (bulkhead, drum and felt washers). I was thinking about ripping it apart to see if the rust area can be repaired and then just replace the felt washer. I really hate have to replace it as it's only 8 years old (with just a 2 person household).

Any ideas? Thanks.