Hi,
Boss White and Hemp has no place whatsoever in plastic drainage, and indeed very little use (to supplement/replace the fibre washer on the trap to waste tail joint) on older systems either. Hemp is primarily intended for sealing tapered threaded joints. It will force open and break fittings if used on plastic threads. In any event, there are very few if any threads which need sealing in waste systems. The threads hold the joints together, and it's the joints that need sealing. Too many people attempt to seal the threads on waste fittings (and compression joints!) to no avail. The joint, and any leak, is elsewhere!
My own method in fitting wastes is to bed the underside of the waste fitting with silicon sealant (a good quality 30-year brand), apply silicon sealant in moderation to the washers underneath, and tighten up. Most washers will be of the hard plastic variety so won't bulge out under pressure. In solid wastes as used in some wash-bowls the joint between the waste and bowl is the one to seal properly. In overflow wastes the underneath has to be sealed as well. It's difficult to give precise instructions for all waste types. Identify where the water will flow, clean and dry these areas, apply sealant evenly and moderately, and tighten up firmly. Clean up, let dry, and all should be wonderful.
Rgds.