using setter will create silvering problems. I never use it. always use softer even on flat surface. if not use softer the decal will have wrinkle problem most of the time.

watch that video careful. there is a lot of brushstroke. hard to see in video but in person, I can imagine how ugly it actually is. also the dim light while the recording session make the stroke invisible. I would say metallic paint is no good for handbrush. unless u r okay with it. but seriously for a expensive kit like sinanju, u better try on cheaper kit 1st to see if the result is ok for u or not.

dont follow Prime92 way of modeling. she don't use primer as base, never dilute the paint with thinner, use straight out of bottle. if u follow that technique I cant imagine how thick the paint is just to get it even

Retarder only help a little to slower the drying time of the paint. It doesnt help to reduce the brushstroke if ur handbrush skill is very bad in the 1st place. handbrush need a lot of practice. therefore I myself skip it. for kit like sinanju I wouldnt risk of paint chipping(coz of not using primer). I'll cry if that happen. I suggest polish ur skill with bootlegs gundam, then move on to ur sinanju after u confident enough.

also metallic paint is very unforgiving with uneven surfaces, scratches. Metallic by handbrush often will make the kit look like dented all over the place, like factory rejected mobile suit.