Exactly what emc said. Having them balled is the most effect way of putting anything onto the skin.
I mean I guess you could try just having quills raised without balling up but that would be hard.
Coming it through wont work either, because your more likely to end up just spreading it over the quills, but you want it under them on the skin.
Really balling up is natural. Holly is super tame, rarely balls up. But she still balls up whenever you wake her so that's when I do the spray on her.
Balling up is natural protective behaviour, to make themselves safe.