Indiana Berry www.inberry.com sells a gizmo for wiping herbicide and it
works really good. I used to spray Round Up and the drift was always
killing other plants too. Now I just wipe and the undesired plant is
history. It works via gravity so would be impractical above waist height.
Something else that works is rubber gloves. Carry a bucket of Round Up,
dip your gloved hand into the herbicide, then onto leaves. It's slow but it
gets the job done.