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The average car has about 80 sq. ft. of area to paint, and might take about a gal. of paint to cover with single stage paint, about 9-10 lbs. a gal.
The B-17, had a 1400 sq. ft. wing, so we're talking 2800 sq. ft. of area to paint for just the wing, it could easily be 7-8000 sq. ft. of surface to paint on a B-17.
So we're beginning to talk about maybe 1000 lbs. or more just in paint on a B-17, plus the dull finish they painted them in had a rough texture that could disturb airflow.