Best to flush it with the old radiator in, so it catches all the junk. You need to drive it to get coolant hot. Flush out all coolant (open block drain) and fill w/ pure water, flush again, then add Prestone SuperFlush or such. I use powder citric acid (ebay), which is the same. Then flush again and run a bit with plain water. Finally, when installing the new radiator I add a screen in the upper hose to catch any debris so it doesn't hit the radiator and take it out after a month or so. I buy stainless mesh strainers at the dollar store and cut them. I recently put Evans waterless coolant in both my 300D's (after removing all water), and no problems so far.