No one expresses any concern about spreading virus by using the dip for successive clumps. When using a liquid dip, place the tubers from one clump in the solution for about 15 minutes. Never dry tubers on concrete, because it tends to draw the moisture from tubers and make them shrivel. Keep the tag with the cultivar name with the clump at all times. With a few containers, one can keep each clump in a separate dip and soak each clump approximately 15 minutes.
Some growers add a systemic insecticide to the dip.
Various authors recommend anywhere from 15 to 30 minutes for the dip. Some experienced growers see no noticeable difference in the success rate using or not using a fungicide. Perhaps a fungicide is more necessary in high humidity climates than in areas with low humidity. Discard any tubers that float in a dip, because they will not keep. Have a small container of water nearby to wet the pencil.