I have read a few posts lately, here and on other forum, on sanding curves- This will not quite do compound curves but will do concave/ convex. I made this about 20 years ago and have used it for all my sanding ever since.
I use a disc sander I made with a motor from a free standing (i.e. portable) evaporative cooler (not sure what you call them there), I picked it up at the rubbish tip for $10. Just pull the ‘pelton fan blades’ off both ends of the motor and you have a low revving 4 speed double shafted electric motor. They are usually, 3 or 4 speed (about 150 rpm max); the motors are sealed, so no arching (because they operate in a very humid atmosphere), the shaft comes off both sides of the motor, so you can put a disc on both ends (fine & coarse).
What do I mean by an evaporative cooler
– ‘ Google‘ it or
Evaporative coolers use a fan or blower to draw in outside air and pass it through a wet filter. As hot, dry air moves through the filter the water evaporates, cooling and humidifying the air. The cooled air is then blown through the room or house.
These are some of the profiles I do, the smaller one is 2" and the larger 3"
The disc is made of "custom wood" not sure what it is called there but it is composite sheet used in cabinetry/ kitchens etc. The material I used is 1" thick . I roughly cut out a circle near the size I needed and fixed it to the shaft. I then made a tool rest and 'turned' the edge with an appropriate size wood lathe tool ( I made one out of a file ground to the right radius).
As you can see you could add another disc on the opposite end and have coarse/ fine. this sander does it all (for me), takes a few to get used to sanding curves but when you get the knack, you will be impressed with how easy and accurate you can be.
Have fun Pete