May I just give you an advice based on my personal experience?
1. Healing brush works nice to clean some skin defects.
2. As healing tool uses some space around actual brush size, avoid using
it near edges of some contrast parts. e.g. close to lips.
3. If some parts of a skin are overburnt, there can be 2 techniques:
using healing brush from the edge of the normal (not over lighten)
skin and moving inside burnt areas
or using clone tool to fill saturated color area with normal skin
and then using healing brush to "flatten" the result.
> I am setting up to try to repair and restore some very old monochrome
> images for a relative of mine. I am fairly confident on how to proceed
> but, I have one main area of uncertainty, namely when to use the clone
> tool and when to use the heal tool. I have Googled until my eyes are
> monitor shaped and I am not really any the wiser. So, I would be most
> grateful if someone could point me towards some readily understandable
> tutorial material, preferably audio/visual, that I can study and
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