You put lead into putty if you want to use it as a caulking compound. And then one customer had the nerve to ask why. We suddenly realised that none of us actually knew. And it took a surprising number of phone calls to find out, since most of the answers we got were of the Well, that’s how it is done vthat’sy. So just in case you want to impress the people in the boatyard, here is the answer, courtesy of Bob Forsyth.
The main reason for adding lead to putty is to prevent it becoming too hard. The seams have to move. Some people even add a bit of grease to make it even more pliable. A second reason is that in the underwater seams, microbes can eat the linseed oil in the putty and the cotton caulking, but dont like either with lead flavouring. So the caulking stays put for longer. And why the choice of red or white lead? If you use red lead in the mix in topsides seams, but have a light coloured topsides, the red colour can leach through the paint and look unsightly. So it is red for underwater, and white for topsides if they are a light colour. Now you know. Snag is, you can’t get white any more.