on my experience, painting metal surfaces such as titan's aluminum brushed finish back casing is quite pain in the a** when you use spray paints in can. since the spray diameter is still big, the paint easily chips off or peels through short time.
I recommend painting metallic surfaces with the ones used in repainting cars, compressor powered air spray. The pressure is a lot higher with that thus the spray droplet diameter would be a lot smaller too. with that, the gaps between spray droplets become smaller, making it tighter.
I did that on my HTC voyager, where the front part of the case has metal coating. The paint lasted as expected. Clear coating is also smoother compared to that of spray in cans. The only downside is it costs higher too especially if you do not have the materials ready.
HTC Tanager(2004),Voyager,Motorola MPX220,Dopod 575,O2 XDA II,XphoneII,XphoneIIm,XDA IIs,XDA IIi,XDA II mini,XDA Atom,Dopod 818pro,Palm Treo 750,O2 Xda Executive,Dopod 838,838Pro,HTC Wizard,Prophet,TyTn,O2 Atom Life,HTC Touch Cruise,TyTn II,HP Ipaq rw6828,Dopod 900,HTC Touch,Touch Pro,Samsung Omnia i900,Omnia II,HTC HD Mini,HTC 7 Mozart,Titan,Desire S,Sensation,Sony Ericsson Xperia X10i,Xperia Arc S,Xperia U,Nokia Lumia 900,HTC 8S,HTC explorer (current)