@ plugp7
painted v unpainted more relates to radiators, you can not solder to a painted surface.
You will not be able to solder the heatsink to the case of the light without running the risk of the light deforming ,you would need that much heat. Good artic paste has really good transfer properties, you should be fine with that.
What heatsink are you using in your build. Trout has designed a updated heatsink assembly that can be found in the following tread
http://www.singletrackworld.com/forum/topic/lumicycle-halogen-led-kit-vote
another option is to do as some people have and put a heatsink for a rc car motor size 540 onto the outside of the lamps to help aid cooling see the following thread for what i mean.
http://www.singletrackworld.com/forum/topic/new-xpg-board-and-driver-for-all-you-home-brewers-out-there
If this is secured with a couple of small screws through the case into the bulk of the heatsink and artic paste is applied between the heatsink and case you will be doing pretty much evrything you can to keep it all nice and cool.
Lastly when you are out on your bike the movement of air past the light will be what keeps it coolest when stationary it will get warmer. When you test it i would not get too concerned if it gets quite warm when you come to use it in reality you will keep it cooler.