Sanding something that is flat has been well worked out over the years.
Shop-based sanders came out in the early 1800s, but it wasn’t until 1927 that Porter Cable released a portable belt sander, called the Take About Sander.
Dremel was next to come up with something new, and released an oscillating sander in 1948, called the Moto-sander
Next was Festo (now Festool), releasing an orbital sander in 1951, the Festo RTK
Then in 1976, Festo came up with the concept of a random orbital sander. These were aimed at the automotive finishing industry, but were very suitable for woodworkers as well.
There is one common concept with all these sanders however. They are all about sanding a surface flat, or at most on a convex curve. I know that is a bit simplistic, but you get the point.
What happens if you want to sand inside a concave region?
If you are using a lathe, then you can use a rotary sander that is spun by contact with the workpiece, such as this rotary sander from U Beaut
But if your object is not mounted on a lathe, what other options are there?
I’ve got a couple (and I know there are others). One is the Festool Interface pad, as I have previously mentioned back in 2012
It is a foam disk that attaches by hook&loop (which we commonly call Velcro!) to the sander, and then attaches the sanding paper to the other side. It provides a cushioned surface that can get into concave curves (and is good for convex ones as well). The one I was using finally gave up the ghost (it is a consumable after all), so I picked up a replacement.
The other solution that I got from Carbatec, is a product from Arbortech called the Contour Random Sander
It fits to your angle grinder, and I am particularly interested in trying this out. It can get into deeper areas and tighter curves, yet still has a random orbital effect, as the sanding disk is free spinning and (just) offcentre.
After all, not all woodworking is about items that are flat!