Lets go through each factor and highlight the most important points to understand how they contribute to the success of watch replating.
factor 1 -
For the finish and luster of a replated watch to last at least 7-10 years or more of regular daily use, you will need a respectable thickness of plated gold. From a corrosion resistant standpoint it will be just as important to have the unseen recessed areas (i.e. threaded areas, lugs, crystal edges) of the watch case to also have an adequate thickness of gold plating. Before we get into that level of detail, lets take a moment and learn about how goldplated thickness is measured within the jewelry industry.
As a starting point it will be necessary to define the "unit of measurement" that describe gold plated layer thicknesses. The most common unit of measurement in the US is the "microinch". In layman's terms, a microinch is actually the same as one millionth of an inch (see chart a).
For Europe and some U. S. industries (i.e. jewelry trade), the unit of measurement is often expressed in "microns." As an example, the edge thickness of a US dime coin is 1250 microns or 1.25 millimeters.
Plating thicknesses for precious metal objects and related trade is regulated by specific rules that are administered by the US Federal Trade Commission. If you are interested in more detail, additional information can be found in the Code of Federal Regulations, Title 16, Commercial practices Part 23 January 1999. Precious metals under FTC regulations include gold, silver, and the platinum metals group. FTC requirements for plating thickness are expressed in both microinches and microns (see chart b).
For an even clearer comparison, chart c describes the average thickness in microinches and microns of some common objects most all of us are familiar with. As you can see most electroplated gold finishes are actually quite thin by comparison.
After comparing chart b and chart c, the first thing one would ask is, how can any gold electroplating as thin as what is designated by the FTC regulations last very long? For watches you would be right to ask that question.
However, bear in mind that in the decorative world of gold watch replating 5-7 microns is quite heavy! Combined with the addition of "alloys" to harden the gold plating, this thickness of electroplating is more than adequate for most replated watches.
Some electroplaters might tell you that 10-20 microns is the correct thickness for a real authentic "Swiss quality" watch replate. If you are replating with a layer of 99.9% fine pure gold with no hardening alloy, that viewpoint might be right. Depending on the watch case design, you would need this heavier layer of fine gold because the softness of the plated gold would start to rub-off the watch's surface through normal wear.
However, with today's goldplating formulas, extremely hard gold finishes with excellent resistance to scratching and abrasion are available. As a result, beyond a certain threshold it is not necessary or advantageous from a durability and "rub-off" standpoint to replate watches above 7 microns.
From a technical perspective, proper thickness of the gold plated layer also helps minimize the impact of "porosity" within the surface structure of the replated watch. We will talk in more detail about porosity and its relationship to premature surface discoloring later in the article.
It is just not entirely true that plating thickness on the high wear areas of the watch determines the durability of the replated watch's finish. Thickness is important, but the overall "average thickness" of the goldplating over the entire surface area of the watch is just as important.
The "physics" of the electroplating process by its very nature has a tendency to deposit the gold on any object unevenly. This is especially true for the intricate recessed areas and contours within watch case design. Prominent edges and significant decorative surface (the exposed areas around the watch crystal and lugs) will receive a thicker plating than the recessed areas within the watch case.
To prevent any surface oxidation and discoloring from propagating from the unseen secondary areas of the watch case, it is very important that a replated watch have adequate gold thickness on every portion of its surface area.
A goldplated layer with at least 3-4 microns in the recessed non-show areas of the watch case is quite effective. This level of gold thickness will eliminate corrosion from starting within the watch. This is especially true for the threaded areas of the case around the removable back cover of the watch and the lugs that secure the watch band. Those portions of the watch's surface are in constant incidental contact with the oils of the skin, chemicals, and perfumes.