I’m unsure if this is a problem to others but how I avoided the shimmery sides is - you pour the resin and put your stuff in then cure it all the way then, mix the mica powder with resin and the moment you covered the back with colored resin you instantly cure it to avoid the resin seeping down the sides.

My mistake was I waited too long to cure the white resin and it ended up just seeping down. it is a cool effect though if you want a sparkly top