If a recipe calls for cake flour must you use it. Can you substiture all-purpose flour instead. What difference does it make to the cake? I am trying to switch over from box cakes to all scratch cakes and this is a question I need to know for cake flour is hard to find in bulk in my area.

Thanks

Cake flour and AP flour are not interchangeable. This recent thread may help:
http://forum.cakecentral.com/cake-decorating-ftopic-582778-0-days0-orderasc-.html

If you must do a substitution:

If the recipe calls for AP flour, you can substitute 1 cup + 2 tbsp cake flour for every cup of AP flour.

If the recipe calls for cake flour, you can substitute 7/8 cup AP flour + 2 tbsp cornstarch for every cup of cake flour. However, your recipe will turn out more dense.

It's really best to use only what the recipe calls for.