In the event you cannot view YouTube (my IT department has blocked it) and you have Wildfire 3, try the following:
In Assembly mode (this is a similar method to what is done in Drawing Mode)-
Select View Manager in the View drop down menu.
Select Xsec (cross-section) tab.
Select cross-section you wish to modify. (sorry, the cross-section must already exist)
Select Edit or right click and choose Redefine.
Choose Hatching.
For each part you wish to exclude from the particular cross-section, choose "Excl Comp".
Choose Done.
Choose Done/Enter.
Close View Manager.