Leaky Radiators?

Over the years I have from time to time removed the radiators in the house to decorate behind them. I've always managed to get them back OK sometimes with just a slight leak around the union of pipe and radiator which has been resolved by an extra tightening of the nut. However after removing the kitchen rad this week and putting it back I just cannot eliminate a very slight leak at either joint on the rad/pipe union, no matter how tight I make the connection. Should I be fitting a new washer or something? The system is quite old, probably 20 years or more. Thanks in anticipation