I don't know about plating directly to zinc. Pennies are made of zinc and they corrode easily. I am also a metal detector enthusiast; most pennies I dig up from early 1980's and back are normally well intact. The one's after that are green and heavily pitted. Which means that if you do not ensure a proper surface preparation you could see your watch corrode. It is best to plate onto either Silver or Nickel. Nickel is preferred since it is much cheaper.
Most vintage watches that have gold content will most likely be brass or nickel underneath. If it is nickel, you can plate directly onto that. Brass, on the other hand, can also be plated directly onto but you run the risk of the finish corroding if you do not prepare it correctly. So again, a coat of nickel is best. Nickel plating is very particular and you must get the surface very clean. Gold on top of nickel will last longer than just gold onto of the brass. So you safe money and man hours later on.
Nickel will plate onto just about anything, but there are exceptions. Now the important thing is to prepare the surface just like you want it to look. That means removing all scratches, blemishes and any surface defects that you feel detracts from the look of the final product. If you want a high shine, you must shine it prior to plating it. When you plate the final product, it will have the same defects and blemishes that you had prior to plating. So the first initial steps are a must for a nice finish. The great thing about playing with watches, is that you can get many at pennies on the dollar and learning to plate on different items is very cheap. There are so many varieties of base metals on watches that sometimes it is a guessing game as to what plating sequences you have to use to get the final look you want.
I replate old gold content watches to bring that brand new look back to them. Again you must prepare the surface and you can just plate directly onto the gold without the nickel plating.
Chrome is another story. You must completely strip off the old chrome and most likely have to nickel plate prior to chroming the surface. Chrome is not cheap so I use what is know as Copy Chrome. It looks just like chrome and is just a durable. The only thing is, it must be polished on occasion. But the savings are huge when compared to real Chrome. You can strip the chrome off with a dremel tool and a 300 and 400 grit sanding sponge. The chrome will strip off quickly. You can also soak the part in a Muric Acid Solution.
To prepare the suface I follow the steps below:
1. Strip off any surface that cannot be plated onto. Like Chrome.
2. Wet sand surface with 400 grit then 1600 grit then polish up with brasso or flitz. You can also use a jeweler's rouge and polish here too. Get the surface to look like you want it!!!
3 Wash the surface with Soft Scrub to remove any oils and dirt. Wear gloves to prevent contamination from this point on. Infact, you might want to wear gloves when you wet sand as you can dry out your fingers and they will crack severely afer a few days.
4. Plate surface with Nickel or Silver if neither is present. Nickel preferred since it is cheaper and more durable. Although you can plate directly onto brass; again, nickel is preferred prior to plating.
5. Wash surface again with Soft Scrub and lighly polish and clean again with Soft Scrub.
6. Plate the surface with your final plate. This is either gold or chrome. With chrome you only need one plating coat as more is extra preparation. If you are plating with gold, plate then rinse then replate and repeat as desired. I normally plate with 5 to 6 coats if that is what you want to call it.
7. Protect the surface with a little polish. Preferrably Flitz as it is non-abrasive. Brasso can be used but dilute it with a little water to prevent cutting into the final finish as Brasso is an abrasive.
This is a quick how to. You can search the internet to find plating solutions and even very good references. I prefer to use www.caswellplating.com, they have a great forum and support for their buyers. If you decide to buy, get the Plug and Plate kit that you desire. Once you get one plug and plate you really only need extra wands and the plating solutions. You don't have to buy a new kit for every type of plating you want to use. As each kit comes with the same power source. The gold solution 4 oz bottle is about $25 and will plate 12 to 18 watches. The nickel solution is $15 I still have a lot left so I can't tell you how many you can plate. My purchases were:
Gold Plug and Plate Kit
Nickel Plating Solution
Copy Chrome Plating Solution
Stainless Steel Activator (This is Muric Acid you can also strip chrom with it)
Extra Stainless Steel Wands for each plating solution
Brush Plating Bandages (you can cut up a cotton tee shirt)
In fact, you can just buy the solutions and make the rest yourself; I am lazy so I bought it all.
Finally most old watches used Nickel as the final plating. Especially Timex and Westclox. It may look like chrome but infact is nickel. Some people will break out in a rash when they come into contact with nickel; that is why they really don't make Nickel plating a final plate these days. Good luck should you decided plate some items. It is very easy to do and just takes a little patience; as with every other aspect of watch repair.