This is how I do them m8, Learnt from experience over the years and works a treat-
step 1= grind to clean metal around hole (do this before step 2, as you'll
won't get grinder on it after, yip learnt the hard way myself )
STEP 2= Tap hole inwards a tad, use old pozi screwdriver or punch
usually do 2 quick taps and its perfect, don't go mad
Step 3 = Weld up holes on low setting in tiny bursts so as not to warp
panel, and let cool for a few secs between each burst.
The reason for tapping the holes inwards a bit is so you can
weld from the side of the hole on solid metal over, to build
it up, if you try and start welding right on the edge of the
hole you are gonna just burn it away.
Step 4 = Now you should have a solid hole, with a bit of weld to grind
down, but as the hole is inwards you will still have plenty of
thickness left after you have grinded it down flat, I always
use flap discs for this now, grinding discs are to abrasive.
Step 4 = Skim of filler and you have holes that will never ever crack