[Sailor Moon] Why do the Sailor Scouts paint their nails before putting on gloves? it would just smudge and ruin the fabric.
Because poor nail care is the purview of the villains. 
Duh.
Duh.
actually
it... was to sell toys
I believe some of the wand/pen toys were also nailpolish
so it was to show MAGICAL NAIL POLISH and then their outfits
it's the same reason why the outers transformations focus on lipstick
The Outer Sailor Senshi, Transform and Attack