When you cut a V groove you are depending on the adhesiveness between the old material and the new material. By cutting a V you increase surface area. But generally it will let go again eventually.
What you want to do is what a dentist does. The deepest part of your groove needs to be wider than the top of the groove. Making an upside down V groove. Now, even if the two materials eventually delaminate, the patch material can't fall out.
