You can get fillers for items you want to be powder coated such as :
http://www.thermobond3.com/
But, as has been said already, a less than perfect repair will still be visible after coating.
As a rule, you can only coat metal, as it is an electrostatic process. Specialist applications to exist for coating other materials, but they are not common.
There is more info on the process on our website for those interested:
http://www.kintech-powdercoating.co.uk/
Although this isn't a plug, as we don't really do non industrial jobs ( unless I am made an offer I can't refuse

)