The good news is that without hardner, the alkyde is much easier to repair.
Assuming it is fairly fresh, carefully press a smooth shiny object onto the thumb print, like the back of a spoon. Just enough to flatten out the margins. Orient the bumper so that the mark is level and horizontal. Mix a little bit of the paint with a slightly higher than recommended ratio of Naptha. Use a small paint brush to fill the mark, making sure the paint is slightly higher than the surface.
Leave it for a couple of days indoors.
The solvent will soften the under lying paint and the edges will blend.
You will have the a slight blemish at most and no one will notice it.
Hint, always use hardener. The paint is much stronger and you can handle it after 24 hrs without leaving marks.
