To remain a bonus action, it should cost a consumable (oil flask, alchemist's fire, poison).
I'd argue that it should be an action regardless - because if it's not, then you're undermining features and perks that specifically let professionals do it more efficiently, such as Fast Hands from rogue (an already marginalised perk), letting you take the Use an Item action as a bonus action - this perk is ideal for coating weapons, which would normally be the "use and item" action.
