I use sparkly gold mica mixed with a little bit of glycerin to 'paint' on the surface of my finished soap. but it's on a very small area- just to accentuate where I've stamped my soap. I stamp and paint during the first week of cure, and it takes a few weeks to dry (see the bee):
As wonderful as the technique works on small areas, there's no way I would ever think of attempting to paint swirls over the whole surface using this technique. That would be very messy and troublesome indeed.
The only 'after-the-fact' mica thing that I ever apply over the whole surface
of a soap is with my Salty Mariner soap. Basically, I take a dry paintbrush, dip it in sparkly iridescent mica, tap off the excess, and then I dapple it all over my freshly cut soap -just to give it an all-over iridescent sheen. But that's as far I ever go.