Hey David. Built a pvc tank like you did and made up 10/90 batch of solution. Started out with a nice black barrel.
3 cycles of 30 seconds each Carding with baking soda 65 degree weather Some areas are dark and weak contrast Some areas are lighter and have weaker contrast (thin bluing perhaps, or over-carded?) Areas on both sides of the rib are quite black
My bluing looked good, but wasn't uniform because I had to fix a rib midstream. Had it been, if I've got good black, but not much contrast, would you think I need more etching cycles?