Whenever I use a body filler like P38, once I've sanded it back I like to apply a coat of spray putty which is basically a sprayable filler but it fills a lot smaller holes than P38 or even traditional wood filler. It's available on Amazon, just search for 'Motip Spray Putty.' Sometimes it is seemingly out of stock everywhere online. If that's the case, Halfords do a good spray putty.
Spray a coat on the area with the 'pot holes' and then sand back with something like a 400 grit sandpaper.
I will say though, it currently looks like you need to do a lot more sanding - the above picture looks way too thick.
