In this picture I've finished sanding the plate surface in both directions. Notice the abrasions about the plate surface. This will be where HHO bubbles form. The better the quality of sanding you do on the surfaces of the plates, the better HHO production you will see. With the exception of the end plates, you'll need to do both sides. Using a small sanding block you can also be taking out imperfections in the plate surfaces so that the surfaces are flatter.

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