I've been prep'ing my concrete patio prior to using HD 1-part epoxy ---I've used their cleaner and the etching solutions (including scrubbing them in with a large scotchbrite pad) and I've rinsed them off with my powerwasher -- but when I wipe a damp cloth over the surface, I still get a light brown residue.
Question - how clean does it need to be in order for the primer to stick and if I need to do more, what is the best cleaner / technique to use?
I haven't painted too much concrete but since no one else is offering any advice:
You can never get concrete 100% dust free simply because concrete is made of things like sand and pulverized stone (dust). if you remove as much of the dust as you can, powerwashing being ideal, you should be able to simply paint overtop of what ever is left.
Keep in mind that you need special paints and primers. You are going to have to look into that further because I am not certain of the paint exactly. I remember when I had to prime the conrete before painting the primer reminded me of a general purpose glue... but as I said look into that.