Valejo Metal Color Issue, Suggestions???

Are you dead set on using Vallejo Metal Color?  There are other brands of metalizers that don't have those issues and don't require you to mess with them to make them work.  Alclad and MRP are both trouble-free.

I have not used Vallejo Metals,but Vallejo Flow Improver definitely makes a difference when spraying Model Color.

Pretty much all my tip dry issues have gone away with all my acrylic paints by pre blendin Liquitex retarder into my thinners. Or a drop or two into the paint in the airbrush cup if pre thinned already. 6 drops of retarder to every 3 oz of thinner blend I make up and thin my paints with.

That said, a lot of people like Vallejo flow aid rather than the thinner if they need to extend the thinning of pre thinned paint. My own thinner blends have both flow aid and retarder in them. Just sayin.