Adding Features

Having Imported all my colours for features I quickly realised some of the spreadsheet was wrong and ended up with some products having colours they dont come in.

I exported these products altered the features to be just the ones they come in and re imported.

I assumed this would overwrite and delete the previous ones but no, It Just added the new colours to the existing incorrect ones.

Is this correct do I have export ALL the features and then delete the feature in admin so then I can re import the WHOLE LOT again even though I only want to update just 20 products.

Given the amount of products I have i sometimes get problems when exporting features so dont want to if I dont have to.