Three years ago I added three used cast iron radiators to my heating system. I caped one end and put a garden hose to the other side opened the air vent and slowly filled it. When full just closed air vent. No leaks and all is good for the last three years. Flushed out the radiator with water. Just make sure your water pressure is not to high. Most hydronic heating systems run on about 12 to 15 PSI.
We buy salvage rads all the time. We re-bush the radiator, fill up and test at least to 30 psi. Set it in a dry spot, Seattle is a little tricky for that! Make sure radiator is dry and look for any wet spots, drips etc. After half hr or so we feel good about it. Give quick flush out first also.