OK.. I'm not sure this is an approved method, but heres what we did to a friends boat instead of replacing whole stringer. Kind of an experiment.
First we drilled 1/2" holes in the 2x6 from the top down about every 2". Keep in mind this stringer was as bad as they come black,soft,and sounded hollow when you tapped on it.
Then we made some sawdust out of some good wood and sprinkled it into the holes and poured in some penetrating epoxy(a wood rot restorer)available from www.westmarine.com (http://www.westmarine.com)
then packed it down with a 1/2" punch and repeated the process until the hole was filled.
When all holes were full(about 6' worth)we triple fiberglassed the stringer and painted his bilge with tinted black resin and it looks like it could have rolled out of the factory that way.
To date only one season has been put on the stringer but it is still hard as a rock.
No warranty implied or expressed... http://free.***boat.net/ubb/biggrin.gif