I'm about to install hardboard (Masonite) siding on an addition to my house. It's 12" wide, installed with about a 8.5 inch exposure to match esisting 1x10s. I've been advised to prime the backsides of the boards before installation for longer life.
Do I need to use exterior primer, or would this be a good way to use up some partial cans of paint that are leftover from other projects?
Anything that will be exposed to the elements needs exterior primer, in this case it would be the cold, so yes, exterior. The idea of backpriming is to create a vapor barrier, so you want the least permeability. To me that would be oil, no latex.