Heating Help

Boiler Water Dark after Radiator Replaced

I replaced a cracked radiator two heating seasons ago with a used one.  The one I got had been in a neglected hot water system and it was full of rusty silt.  I banged around on it with a rubber mallet, dumped it out repeatedly, and blew it out with compressed air quite a bit before installing it.  It seemed that I had most of the loose stuff out before I put it in.

Since then my boiler water has become more dark brown and rusty than it was before, even though I flush the boiler every fall and blow it down a little every week during the heating season.  I'm guessing that the radiator is still shedding rust.  The boiler seems to run fine, and I haven't seen excessive sediment in it when I've flushed it out . . . just coffee-like water at the bottom.

Is there cause for concern? Will this eventually end on its own or should I be shopping for a better radiator?  . . . or am I missing something important?  (The feed water is fairly benign, Lake Michigan sourced and fairly soft.)

Haven't tried it yet

But Rhomar makes a product that should be able to handle rust issues. It seems to be highly recommended on the wall.
http://www.rhomarwater.com/products/residential-steam-system/