I put the chassis back on its spit and gave it two coats of the same paint that I used for the cab - International Interthane 990; its a two pack polyurethane and very easy to work with. The blue is my own mix which I call Sabler.
The spit allows the chassis to tip to and fro to get all the angles but it is bloody hard not to miss bits. I went round and round it from every angle and still noticed two or three bits that need more paint later in the day.