The paint guy at a local O'Reilly's told me that the recipes for their acrylic enamels call for a certain amount of clear to be mixed in. I don't know how much but it soulds like putting more into the final coat should work.

'64 swb stepside (gone)
'57 1 ton
'53 phone truck
'59 swb
'46 1 1/2 ton
'68 swb gmc