I have exactly the same
problem on my bonnet. The car came with a big chip out of the paint just above the central "nostril", the bit with the S
badge on it. It had been patch-painted to stop rust, but still looked unsightly. I filled the depression with body putty, smoothed it with fine sanding sticks then painted with a Halfords chip and scratch system paint.
As I'm a plastic modeller in my spare time (so: not recently then) I have a range of foam-backed sanding sticks by
Flex-i-File. However, similar items are available from Boots, for nail filing/polishing purposes. Graded from coarse (suitable for sanding off old rusty bits) to extra-fine to polishing sticks.
To finish off I used one of those scratch removing kits you see every year or so at Aldi or Lidl. The finer of the two polishing compounds took out the last remaining hazing of the finish.
The patch looks a lot better but still doesn't match the rest of the bonnet, since the metallic paint has swirled rather than being laid down flat. I might cut it back and try using the Halfords paint, thinned with cellulose lacquer thinner, in my airbrush. I can't think of another way to achieve a match with a metallic finish (Arctic Grey, S7N, in my case). Otherwise it's going to be Chipex.
If you've got any rust, use Kurust or similar first to prevent it spreading, then paint it.
