A customer recently asked, "The metal elbow under my bath tub is leaking. Should I seal the leak or is this something for plumbing contractors?"
There are many many products are sold to seal leaks in plumbing pipes, however, the directions want the material cleaned thoroughly to allow the epoxies to stick to the fitting. With the amount of work this takes, combined with the cost of the repair product, it's probably better to just replace the elbow with a new piece. If not replaced, the leak will eventually find a new location alongside the putty.
If you have a leak, the problem doesn't just occur where the leak develops, but over the entire inside of the fitting. The pipe is most likely thinned by age and harsh acids (chemical drain cleaners) and is deteriorating.
So in this case, the licensed plumbing specialist is the answer, not the DIYer.