When to trade for resources
A common mistake made by new players is to trade for a resource whenever there is a deficiency. This is not always necessary and can result in a player wasting too many civilian factories on trade. To see the impact of a deficiency in resources hover your cursor over the progress bar showing how much of something you are producing (see picture directly below).
A good rule of thumb is to trade so that you have less than 8 of a resources needed.
Due to the lack of resources there is a -30% reduction in the production efficiency of the Close Air Support. Additionally there will be a reduction in the growth of the production efficiency, this can be seen by hovering over the progress bar to the right of the graph icon.
As you can see in this image the -10 aluminum is resulting in a significant loss in the production of Close Air Support. In this situation we will trade one civilian factory for 8 aluminum. Although this will leave us with -2 aluminum you will see in the following picture that this does not create a significant impact.
By trading for 8 aluminum we have reduced the the -30% reduction in production efficiency to -2%. It is not worth to trade for 16 aluminum to remove the -2% debuff.