Thanks mangsculpting with putty really is one of those things that you can spend a ridiculous amount of time on. In order to get the side air vents nice and smooth/angled I sanded them using 600 grit paper, primed them then very lightly sanded with 1000 grit paper (did those last steps X3 before it looked the business).
One thing I find strange is the Tamiya 1000 grit paper has a lot more cutting power than the 1000 grit paper I bought from Bunnings (they look totally different too). You certainly have to be very careful when using the Tamiya stuff, don't use it when tired is my advice!