You should prune arborvitae hedges so the base of the hedge is wider than the top to insure adequate light penetration to the bottom of the hedge. The lower portion of the hedge will die if it does not receive enough light. Prune your arborvitae in late winter or early spring before new growth begins.  Arborvitae will withstand heavy pruning and shearing because new branches develop from concealed buds in the branch crotches. Arborvitae can produce new branches from concealed buds in the branch crotches. It is best to never wound the bark of any tree. Tree bark will seal over but the wound will never heal like our skin does.  Most pruning sealers should be avoided. Wounds in the trunk increase the risk for insects and diseases. This site has information about your tree.https://www.mortonarb.org/trees-plants/tree-plant-descriptions/eastern-arborvitae