I am sure
this has been done before, but maybe it's worth mentioning
The other day, I was working on a sign, and while moving the
stool I was sitting on, I smacked it into the sign and put a
nice dent in it.
I remember my father teaching me the trick of using a steam
iron, and a wet cloth to "steam" a dent out of wood.
Well it worked on HDU just fine.
Here is an example.
I used an iron on high (max setting), and a fairly wet rag
(almost sopping wet).
This was steamed for about 10-12 seconds.
It works great on small dents and dings.
Sand to blend/feather if need be.
If the dent won't come-out, then it might be a "gouge" with
Here is the dent.
Steaming the HDU.
as the structural integrity, I really wouldn't be too concerned.
Were talking little dents and dings (nothing deeper than 1/8" or
so), that once primed and painted will be just fine.
I like it a lot better than trying to fill/blend an area only to
have the repair visible after painting. This method is pretty
much invisible, and quick.