A corner of my 18' cc o/b boat got knocked off. It's at a spot aft where
the gunnel, transom, and flat top side of the gunnel all come together. The
spot is approx 4" diameter.

I'm fixing to stick some auto bondo or similar on it and then mold/shape for
the right contour.

The question is how to be able to smooth it on with minimum surface
roughness. There used to be a number of tricks for doing that, such as a
paint stirrer paddle kept wet by dipping in water. Is this still the
easiest way to be able to soft mold the patch and get it close to where it
needs to be before I start shaping when dry?

If I need to, I can finish it with glazing putty, before painting.