Hi Dave
For the chassis, I would use an Enamel. If you can get a heavy duty industrial Enamel that would be the go. I buy mine in 1 litre tins and thin with 20% thinners and spray through a gun. Once it is dry, job done.
If your going the spray can route, I would be leaning towards a Watyll Kill Rust. Its about $20 a can, but it is not to bad. Even though it says it can be sprayed straight on you should be priming.
With a bare metal surface as you have mentioned, start with a coat of Etch Primer. You can then put a grey primer over the top if you desire, but you can spray straight over the Etch. You can get Etch in a spray can from supercheap or any auto store.
Cheap Enamel spray cans, dont have a good build quality and dont offer a particuarly hard finish.
Some or most automotive acylic paints require a clear top coat. The colour is simply a base. You can put an acrylic clear lacquer (nitrocellulose) which is available in a spray can, over the top. However the finish is only about medium hardness and required buffing with polish to obtain a shine, or it just looks like a Satin Finish. For best results use a 2 Pack lacquer like UPOL. It drys to a high gloss and is very durable.
The other option is to put a Green Hammertone finish on, like used on the older scott bonnars. You can get this in a spray can.
