Coal tar has been in use in the Dutch canals for a 100 years successfully.
It is still in use. Your risk to corrosion is on the inside, not the
outside. Even there tar works very well unless it is exposed to oil or fuel.
I have seen examples of steel hulls immersed in salt water unpainted and
suffer 0 corrosion except at the air/water margin. Condensation on the
inside is a whole different matter. There the best protection is urethane
foam above the water line and tar below.
Steve
wrote in message
...
Hi,

I'm considering buying a houseboat with a steel bottom. New steel
was put on 4 years ago, and treated with coal tar epoxy, supposedly.
I was told that the coal tar treatment will last for a long time. Can
anyone tell me if the coal tar treatment is really good for a long time,
like 30 years or something? If it is, then why doesn't everyone use
it? It it's not, then how long is it good for, what to do, and when, in
the future?

Thanks for any help!
David