...they produce a speckled copy even when set for dark...
I deal with this problem by retouching with a paint marker, after
the ink is on the metal. On the other hand, I've never found a
copier that left this unnecessary... Anyway, if you do this, use the
finest-tipped paint marker you can find, be prepared to scrape off
any that gets where you don't want it, once it is dry--I use a
dental tool for this--and use a colored pen, not black, so you can
see just what you've done.
You can also use a paint marker by itself as a resist for free-hand
work. It may pit a bit, though, or come off prematurely on a deep
etch. You can get it, or any resist, to adhere better, in my
experience, by etching silver for 1-2 minutes before applying your
resist. Rinse thoroughly, pat dry, don't touch! When it is dry,
proceed as always. Copper and brass don't seem to need this extra
step. Good etching!