how can i fix my leaking central heating radiator pipe?

I have a slow leak on a central heating radiator pipe coming from just under the regulator/ join pipe. I've tried sealing the leak with a sealant which slowed it down but didn't work. The tube says that it is not necesasary to bleed the system and it can be applied to wet or dry surface????? I don't think so. Can you help with any advise please.