Tamiya grey primer

[Einion]

Brushing off the surface with a stiff brush was one of the things I was going to suggest. For one step more you might want to think about getting a fibreglass eraser. Fine steel wool (0000) can be used gently too. Still won't end up with something as good as it would be if the spraying had gone properly, but it's a rung above the alternative.

Einion

 

[mickc22]

you could also try Halfords leveller, but be very, very sparing with it, do a test on something old first