Best practice when removing a tree branch is (a) not to leave a stub but to cut as far as the ridge or collar where the branch meets the tree, and (b) not to bother with sealants, wound paints, etc. The best you can do is remove the stub as far as the collar and then just hope for the best.
To quote from this link: https://www.rhs.org.uk/advice/profile?pid=233
When removing larger limbs, make an undercut first about 20-30cm
(8in-1ft) from the trunk, and follow this with an overcut. This will
prevent the bark tearing, leaving a clean stub when the branch is
Then remove the stub, first making a small undercut just
outside the branch collar (the slight swelling where the branch joins
the trunk), followed by an overcut to meet the undercut, angling the
cut away from the trunk to produce a slope that sheds rain.
Avoid cutting flush to the trunk as the collar is the tree’s natural
protective zone where healing takes place.
There is no need to use wound paints, as they are not thought to
contribute to healing or prevent disease. The exception is plums and
cherries (Prunus sp), where wound paint may be used to exclude silver
leaf disease spores.