UPDATE.

Further to my last update I have some important news to share. That post mentioned that I was undertaking the treatment of some new zinc plated screws with vinegar to remove the zinc to enable painting.

As I have done for some years I soaked two batches of screws in vinegar. However even after 7 hours soaking including a change of vinegar midway through, the results were not acceptable. I found only 75% of the zinc plating had gone.

Seeking an answer I referred to the interweb.  It seems that the thickness of zinc plating and therefore the time it takes to dissolve can vary considerably. However although during my original research some 7 or 8 years ago the method of choice was vinegar for removing zinc plating I now found that opinion has moved to the use of citric acid granules. These are available in small quantities from Wilkinson specifically to remove limescale from kettles, or in larger quantities from Amazon for use in baking. The data on the various references on YouTube etc suggested these crystals had less smell than vinegar, were quicker to use and were more effective because the acidity level is higher.

Therefore I purchased a small box of crystals from Wilkinson to experiment with. The results are spectacular. The original batch was completely cleaned of zinc in less than 20 minutes. The smell, although still not good, was much less cough inducing than with vinegar. I have since used the rest of the crystals to clean another five boxes of fresh screws, one of which is shown below.

My "recipe" is to half fill an ice cream plastic box with warm water, mix in two heaped table spoons of the crystals,  add three boxes (600) screws and leave for a good hour, just stirring every fifteen minutes or so. Then I wash the screws in a sieve under the garden hose and dry with a hot air gun. The results are great and so much better than using vinegar.

The photo below illustrates this clearly. The box on the left contains zinc plated screws. The centre box is after treatment with citric acid and the box on the right contains "self coloured" screws.  

Following treatment the screws are an exact match to "self coloured" but on drying take on a yellowish hue due to flash rusting. This is perfectly acceptable to me as it helps provide a key for the paint.  This will be my go to method of treatment from now on. As mentioned before, woodworkers use this method to age modern fittings. If the dark gray look of self colour is required, after rinsing, just spray the screws or whatever, with WD40. This drives off the water and stops the items going yellow with rust.