[Mike 85]

Here's the "deal".
You try and fill any "hole" with filler,you got several issues to deal with.
1. You fill over the "area" and block it down. Just how much filler do you think is left holding your "plug" in? I assure you,not much.
2. You can hammer this "area" down so's to apply enough filler to have some integrity to it along with it being "flat". Doable on a cheap out fix.
3. Problem with 2 is,your filler is OPEN on the backside to moisture absorbtion and even possiably breaking off inside and your "plug" becomes a sponge or worse,falls off.