I have some significant industrial experience using paint laden with zinc. If applied properly (so that it sticks to what you apply it to) then a cold application coating can work quite well. The key to longevity is building up enough coats of zinc. The more zinc, the longer it will last. I have done extensive ASTM salt
spray chamber testing to compare zinc rich spray and liquid paints to hot dip galvanizing over black steel
and the key to longevity is coating thickness. Typical galvanized sheet metal is usually applied between 0.9 and 1.2 oz/sq ft, hot dip galvanizing is usually much greater, 2.4 oz per sq ft or more, depending on how slowly it travels through and is pulled from the kettle. If you build the spray coating to a coating thickness of 2.4 or greater than 2.4 oz per sq ft you will have a pretty good zinc coated part, comparable to typical hot dip galvanized coatings. Not all spray zinc or liquid paint zinc paints have equivalent amounts of zinc. Check the label or contact the manufacturer and do the math. For example, if the contents weigh 16 oz and is 90% zinc and you coat 1 sq ft with the entire contents of the can then you have applied 14.4 oz per sq ft, over 10 sq ft this same can would supply 1.44 oz per sq ft. Keep in mind, regardless of whether it is sprayed or dipped, strong acids or alkalis will attack the coating and reduce its effectiveness.