There are lots of variables here. The type of cut (heading vs drop crotch), the level of vigor on the branch being cut, season, etc make big differences in how the cambium fills in to seal the wound.
bsbullie is correct that wound dressings are not needed (and in some cases may even promote decay). In addition to myriad other articles, here's one titled
"The Myth of Wound Dressings" written by Dr Chalker-Scott.
Moreover, even if the tree successfully heals over a wound (judging by outside appearances), the interior of the tree will oftentimes seal off a giant portion of xylem, making it unusable to the tree. Chapter 4 in Dr Edward Gilman's "An Illustrated Guide to Pruning" gives expert insight into the healing process and is worth a read.
The best way to promote healthy healing is to use the drop crotch method of pruning.