I am not familiar with your Johnson TD20 but crank case seal would be typical as on other engines. On my split case Mercs for a trouble free seal both halves need to be very flat. With age they tend to change from original machining especially at the bolt holes. I use a machinists straight edge and check flatness on case and block with back light. If high spots I flatten them and recheck. On the Mercs the crank case cover usually needs to be trued. Usually very little flattening needed to true up and not upset the original line bore center. I use Permatex anaerobic gasket maker (Mumbles uses the it, I use their anaerobic gasket maker same chemistry different viscosity) same brew as Locktite 518. I clean surfaces with alcohol and apply a veeeeery thin layer on each surface by dappeling with tube and spread out with finger (with latex glove), stuff is red and the color on would be uniform and very light red if applied right. Too much sealer will result in too thick of a film and will upset the original line bore of the crank shaft. If there is a lot of squeeze out there was too much sealer, no good, in this case more is not better.
Surface flatness seals, the sealer just fills in the slight imperfections and scratches. I do a dry fit to make sure all parts aligned, then clean and apply sealer. I keep halves slightly apart (to prevent too early a start to set of the anaerobic sealer) until all bolts and nuts are on loose then push together and torque in 3 steps. I let cure a few days before start. Results vary depending on condition of parts and application.
.
