A few tips:
-Paint the hangers and baseplates separately
-cover the axle and kingpin with blue painters tape
-use spray paint specific for metals (rust-stop, enamel based, high gloss: ask at a hardware store for anything like that if you are unsure)
-multiple light coats of spray with proper wait time in between, you wanna dust the trucks with paint not hose them
-after your final coat wait for the full cure paint time
Optional:
Since you are painting the trucks you can shape them beforehand if they are TD or Chinas to take off any molding imperfections or make them look a lot more like actual skateboard trucks. Also, if they came pre colored then you should rough up all the surfaces with steel wool before painting and remove any bits that might flake off.