Powdercoating is a finishing process wherein a powder is sprayed on the parts and liquified through baking then hardens when cool to a tough paint-like coating. It has advantages and limitations. For carbon frames and bonded frames that are glued together, powdercoat baking could damage the frame outright, or render it unsafe.
There is some talk about use of infrared baking to cure powder coat from the outside, reducing heat into the underlying material . Such systems are prone to shadows and hot spots on complex shapes like bike frames. We are skeptical that any such system could be trusted with your safety.
For welded aluminum frames the answer is more complex. Many welded aluminum frame are powdercoated at the factory, they should not be powdercoated AGAIN for refinishing.
Aluminum frames are use heat treatment after welding to achieve the optimum strength, rigidity and crack resistance. The process varies with alloy and it's intended application. While most heat treating of aluminum occurs near 1000 degrees F, the final process is "artificial aging" in the 200 to 350F degree range for a specified length of time. Powder coating requires baking in this same temperature range. During original manufacture this is taken in to account, so powder coat baking can be final heat treatment. Heating again during powdercoat refinishing extends the process beyond the intended specification making the frame brittle or lessening it's strength.
High quality bicycle frames are made of 6000 and 7000 series alloys. Of the many aluminum alloys, these are most affected by extended artificial aging. It's possible that very overbuilt frames of lower quality alloys are less affected but we recommend against entering that grey area!
So, CyclArt recommends that aluminum,bonded and carbon frames only be be refinished with paint that bakes are less than 200 degrees like our Category 1, 2 or 3 finishes.
Technical data on this subject can be found at: http://www.mlevel3.com/BCIT/heat%20treat.htm