That got your attention.
Following up on several owners observations about rusty tow-balls last year I decided to take a two two ball cover approach. I have a pivoting towbal on my Audi with an open/slotted cover and as such the ball seems to rust very quickly. Over winter I used a standard Alko rubber cover and a wipe of grease. I've just pulled the van out to take it for it's service and when hitching up found a light wipe was all that was needed to degrease and find a sparkling ball that did not need any attention with the emery cloth. When away I'll use the other cover but revert to the grease and rubber cover when the van is not in use.

Colin

I have also found rust an issue when using for the first trip of the year. I did consider a plastic bag but thought it might sweat. When I put the towball up for the last time last year I smeared with vaseline and used an old Alko rubber ball cover. This year when taking off I had minimal rust. I wiped dry, gave a light emery as normal and then a squirt of brake cleaner on a tissue. The cover has now been replaced with a dry clean one and I've only needed a few seconds with emery tape on the ball prior to the usual brake cleaner wipe in our 4 weeks away so far.

Colin