Zinc Barrel Plating Current Density

A. Alan, per Faraday's Law, at 100% efficiency it would take 96,485 Amp-seconds (a Faraday) to deposit one gram molecular weight of zinc (that is the atomic weight of zinc divided by its oxidation number), which is 65.38/2 or 37.69 grams of zinc.

Since the density of zinc is about 7.14 g/mL, one Faraday will deposit 5.28 mL of zinc. If you have a surface area of 15,000 x 0.16 = 2400 dm^2, that's 240,000 cm^2. So one Faraday will deposit a thickness of 5.28/240000 or cm, that is 0.000022 cm or .22 micron. So for 0.8 micron thickness you would need 0.8/0.22 or 3.64 Faradays or 350,850 Amp-seconds. At 300 A, this would take 1170 seconds or 19.5 minutes. But efficiency is not 100 percent; it might be 50 percent for cyanide and 75 percent for acid zinc. So the plating time would be maybe 30 to 40 minutes :-)

But you might want to double check some of your units because they don't sound realistic to me. Perhaps your 0.8 µm thickness should be 10X thicker, or 8 µm -- which would be a very common thickness specification. And I have a hard time believing the barrel holds 240,000 cm^2 (258.3 ft^2) of work; 24000 cm^2 sounds more like it. But if your thickness is low by a factor of ten, and your area is high by the same factor, the answer is the same -- a reasonable 30 to 40 minutes of plating time :-)

Good luck.