I bought 14 of those planters one year and painted them up after making 2-row planters out of them. If you lift the rubber up off the wheel make sure you blow out any sand that may be in between the two. I never worried about keeping the paint off the inside of the rubber like you are mentioning. But I would apply some plumbers grease with a paint brush to the inside rubber. This will wash off with a garden hose after the paint is dry. I think it will save you a lot of time and headache. I always set my press wheels up with a rod thru them and positioned the rod on two jack stands. This way they can rotated while you're painting them. Be sure to turn your spray gun down on volume not on the air side but liquid side. You'll have a lot of spots that will be hit twice or more due to the small areas you'll have to paint. Just my experience.