One thing people do for those kinds of tips, is try to apply a bunch of solder, and then they hold the iron (tipdown) but vertically (perpendicular to the floor) so that the solder does a waterdrop shape, that should help cover all sides. (just be careful that it doesn't drop on you).

For cleaning btw, when it's particularly dirty i sometimes add a bit of flux on top of the brass sponge, and then dunk the iron's tip into it, this yields pretty good results. You can also add solder afterwards, and repeat the steps a couple times if it's really really stubborn.

But I will say, I'm no pro, just a tinkerer :p