Are you brush painting or airbrushing?
Metallics just don't brush well, Acrylics are not great generally for brush painting and to get a decent finish you'll need at least two thin coats.
When airbrushing, thinning is absolutely essential, if brushing it can be beneficial, ambient temperature can affect the drying rate so if painting in a warm room your acrylic paint will dry very quickly, thin only slightly if brush painting paint one coat, don't worry that it will look streaky and messy, let it dry, at least a few hours and re-coat, build up the coats gradually.
You also ideally need to wash the part prior to painting and ideally prime it, this could be done with spray can primer or enamels brushed, or airbrushed.
For a good metallic acrylic, try Citadel, the games workshop paints, a bit pricier than other acrylics but very good.
Enamels give a better finish overall, especially for brush painting if you have this option instead.