Engine paint has no gloss
1. What kind of base coat are you using ?
2. What's the air temp & humidity like when you're painting ?
3. Have you tried colour sanding & polishing ?
The trick I've found with air cleaners ( and other surfaces / items , which I won't railroad to right now ... ) is to :
1. Make certain that the primer / base coat is reasonably smooth via careful wet sanding with 1200 grit
( an egg shell-type lustre to it , like a satin finish )
2. Wash part(s) and let air dry until ALL moisture is evapourated
3. Lay down some mist coats ( tacky ) and let cure for ~1 minute or so
4. Gradually build-up colour coat until the part's almost completely covered ; let setup for about 5 minutes
5. Final ( top ) coat should be heavier , without runs , drips , or fish eyes