How to fix leak at PVC/galvanized stubout
Are the threads still good on your galv. stub? 

I have a trick where I use a bastard file to "flatten off" the front end of the stub-out, then run my 1 1/2" pipe die on to chase the threads, then use a rubber slip-joint washer instead of a plastic one.

The reason for using rubber is that the plastic washers rely on an even squish provided by the washer pressing into the i.d. of the old galv. pipe, which is often out-of-round due to age/corrosion. The rubber washer just butts against the end of the galv. pipe and the squish is provided only by the slip-joint nut. 

P.S.- slip-joint connections need to be tightened with a wrench. Folks are often mistakenly instructed by Big-box teenager that "hand tight" is sufficient, it is not.