just the hub is needed as it's that rusts, if the rim makes good contact you may actually see the pattern of the rims mating surface on the hub in rust, you can pay attention to those areas. I usually just use the brush it comes with to apply a small dab and then just smear that around. If you like to over do it a bit, there's no harm in doing the entire hub surface, just don't gob it on. The stuff spreads quite well and should be spread as thin as possible over the surface (for a few reasons, one of which is the sling you encountered already).
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View my fuel log 2015 Micra SV automatic: 7.1 L/100 km ... 39.9 mpg (Imp) ... 14.1 km/L ... 33.3 mpg (US) ...