Questions about painting...

It is a little hard to say - if you can provide information such as : the exact Rustoleum product you used, the brand and type of latex paint, what temps are your radiators actually getting to (ie is this a hot water or steam system), does this have a chemical / plastic smell or is it more that 'heat coming on for the first time of the year' / burning lint smell.

The cast iron metal itself should not emit any smell when heated. However, anything you put on it - primers, paints, strippers, sealers, cleaners, etc will have some period of 'out gassing' at each temperature. That is to say when you put the primer and paint on, you probably smelled paint for a couple days at room temp and now that smell is gone. As the radiators come on for heat, more out gassing will take place and you will smell those chemicals for a while, then it should go away when the paint finally 'bakes out' all of the volatile components at that temp - if for some reason the temp goes even higher, more volatiles may bake out and you will get a smell again.

This is fairly common with new wood stoves, as well. But you should finally reach a point where anything volatile has baked out for the given temperature range that the system operates at. I doubt that you are operating at any kind of temp that is physically breaking down the paint ie 'burning it' Also, the smell may be more noticeable if you have several 'new' radiators operating at once versus just one or two scattered throughout the house.

I've always used Rustoleum primer and Rustoleum spray paint for the finish coat. They smell for a few days and then either we get used to it or it goes away. I suspect the latter, since I've been doing this for years in several houses and never had a problem with lingering smell. Of course, the first couple of times you fire up your system, you're going to smell whatever has accumulated on the radiators over the summer baking.

I don't have any experience with the latex, so I can't say.

As a practical matter, if you have small children I'd say anything that covers and stabilizes the lead paint that's probably already on the radiators is a good thing.

Your radiator temps will never reach 200 degrees. You boiler water shouldn't be that hot (180 is more like it), and by the time the water hits the rads, it's even cooler.

Some people are more sensitive to chemicals than others. If you're worried, I'd call the company for more information. But if you believe what it says on the can, then you should be alright.

I read your post about painting your cast iron baseboards.
I had to replace a huge (23 feet) entire leaking cast iron baseboard and I found the website for the manufacturer so my heating contractor could buy matching. Sorry I don't remember the brand but top line and very expensive.

The section came in a grey color and the directions FROM THE MANUFACTURER about painting said ONLY use oil based paint.
In the past I had painted the same way you did on a small section thank goodness...rustoleum primer and then latex paint.
Eventually rust eventually showed through in spots. And that huge 23 foot section sprang a leak.
So according to the manufacturer, I used oil base primer and then oil based semi gloss and it still looks perfect after 3 years. No rust coming through.

So if I were you - for any you haven't painted yet - use all oil base.
I had NO odors when my furnace kicked in. None at all.
I'm suspecting the latex in your situation.
My understanding is the latex will encourage rust to form.