Re-installing a Mac with an older OSX

TL;DR To re-install an older OSX on a newer one, you'll first need a newer OSX install media to wipe the disk.

The workflow for paving a Mac with an older version of OSX than it's currently running is this:

• From an external medium, run an OSX installation that is at least as new as the OSX currently running. An older installation won't start.
• Start Disk Tools and erase the Mac's hard disk. Use the Journalling file system.
• Boot with the older OSX installation media inserted and the Alt key pressed.
• Install the older OSX.

 Frustrating when software doesn't let me Do Dangerous Stuff :)

Next time i'll just use dban. Let's see how that goes.