Depends upon what was used as the filler. Better practice used in auto-body is to fill those dents with solder rather than Bondo. This is a stronger and more heat-resistant solution and it will sand flatter. Sanding a Bondo-filled dent can cause depressions because the filler is softer than the surrounding metal. Not an issue if you use a hard aluminium block larger than the dent as a sanding-block.
Bigger problem is the heat used in power-coating. It may cause the filler to outgas and cause bubbles under the powder-coating. Personally, if it was small nicks, I would remove the filler and use silver-solder to fill the gaps. Then file and sand to blend into the surrounding tubing; you won't even be able to tell there was an imperfection there. Then powder-coat.
Last edited by DannoXYZ; 05-03-10 at 01:11 PM.