I had to remove radiators in my home for repairs to walls.. I need to put them back.. I want to know should I use a sealer on the threads or tape or nothing? It is a hot water system

Is this copper or steel pipe? I'd use white teflon on copper but for steel use a paste.

A agree with ballengerb1 but the pipes are steel/black iron get the good pipe dope paste. Pipe dope paste or teflon tape acts as more of a lubricant than a sealer. This is so the pipe threads can screw into the fitting better and thus prevent the leak. With black iron or steel pipe if you have a leak many times you have to disassemble everything just to fix that leak that is why it is important to use good dope and make sure your joints are tight.