My company has some parts where just the exterior surfaces are powdercoated (ie tubes) and some where all surfaces are powder coated (ie stamped steel parts). The powdercoat weight sometimes increases the weight of a part by up to 10 percent, so it can't be ignored.
You can do the ol' shell outward trick mentioned here 'Shell outward' results in deleted faces of any face I select.
Then you can apply the powder coat meterial properties to the shell body.