I love my old radiators and find the idea of ripping them out just to repaint them to be a little ridiculous. Ditto putting in forced air heat AND the "professionally painted" old (now nonfunctional) radiators just for "looks". Since when is "looks" more important than having a good healthy source of heat in your house??? The suggestion was made to determine the facts and to then stick with the facts and I wholeheartedly agree although I seem to have reached an antithetical conclusion.
Any homeowner should begin with the basic question of how to best heat the house. Radiators get my vote and there is no close second except maybe hot water baseboard heat. Forced hot air is just plain awful. It's awful for the human body and for the furniture even if it is working right.
I don't mind if the water in my radiators is 30 years old or not. It has never caused a problem and maybe it's even older than that. I don't drink it, touch it or breath it and it doesn't clog up, so what's the problem? Very few bacteria can stand to be heated to those temperatures (except in Yellowstone) and if there is rust in the system, the system seems to have come to terms with it somehow.
Dismantling an entire radiator system just to paint it sounds crazy. All of the radiators I have ever seen have had stuck and/or corroded valves and one would be asking for trouble to do any dismantling. If it ain't broke, don't fix it....
If too much heat comes out of a particular radiator, try wrapping it in something to insulate it and reduce the heat exiting at that particular point in the line. Something like a space blanket. I wrap cardboard in aluminum foil and place this behind my radiators so that they don't lose heat to my (uninsulated) exterior walls. All of this is hidden by my radiator covers.
Which gets back to the paint and "appearance" issue. If the homeowner doesn't like the old paint on the radiator (I spray-painted some of mine and they look great) he could build covers around them. Most of mine are surrounded by either metal or wood and metal covers and this works very well, plus the cats love to lie on them and look out the windows. I envy them. The covers are all different, with some of the covers appearing to be very old and some having been built by the previous owner. All can be easily pulled forward into the room so that dust and lost cat toys etc. can be removed. Each cover has a metal grillwork covering the entire front so that air can circulate easily through the radiator. The grill in each case is painted to match the room walls and even the old ones look great. Heck, even the homemade ones with varnished high quality plywood around the grill look great.
Please rethink the idea of replacing wonderful functional radiators with anything and think long and hard before removing them to paint them. I only witnessed one radiator being "removed" and it was so heavy that it had to be sawn into many pieces first. Have you tried lifting one?
I have two separate circulators which feed two separate parts of my house and it costs me very little to heat my home because I don't mind keeping part of it quite cool. I also supplement the warm part with a woodstove. In my book anyone who is lucky enough to have real functional radiators in their house and doesn't appreciate them doesn't deserve to have them in the first place. These are the kind of people who also replace their entire lawn with sod.....