Chainplates carry out the stressful function of connecting the stays to the hull. All of the force placed on the stays is transferred to the hull via these metal plates. Chainplates come in a variety of materials, but the most common materials are stainless steel and bronze.
In the world of stainless steel, 316L (the L stands for low carbon) is the ideal material for chainplate fabrication as they suffer the least from corrosion.
When your chainplates succumb to the effects of crevice corrosion and stress fractures, it is time to replace them. Sailboat manufactures mass produce their chainplates at the time of production, but older sailboats need to have custom made chainplates fabricated as they are no longer in stock.
To manufacture your own chainplates, you need to locate the raw material: 316L Stainless Steel. 316L is readily available in various shapes and sizes, but for chainplates, you want to use "bar stock" or "rectangular". If you have the choice between the two, opt for "rectangular" as all the sides are sided and the metal is actually rectangular. "Bar stock" is mostly rectangular, but the edges are not completely square and you will spend a lot of time surfacing the metal instead of making your chainplates.
When you receive your metal bars, the first thing you need to do is polish them. A smoother surface is easier to spot fractures and cracks as well as being eaiser to keep corrosion at bay. To polish the metal, begin with a random orbital sander with 40 grit paper, then move up to 80 grit, then 120, then 220.
At 220, you should start to see a faint reflection in the metal. The grain of the metal will still be very present at this point but the metal will begin to shine.
You could continue through finer and finer sand papers, but I like to switch over to a wet stone at this point. Be sure to keep the stone lubricated with a light oil such as WD-40. This will reduce friction and clear the stone of metal dust. If you work dry, the abrasives will clog almost instantly. When the oil begins to look black, I like to wipe it off and start again with clean lubricant.
After the stone, I follow up with 2000 grit sandpaper lubricated with oil. This will produce a good polish which will work as a low quality mirror. Once the metal is "as polished as it can" with oil sandpaper, I switch over to water and 2000 grit sandpaper. A gentle stream of water running over the bar will avoid the black debris characteristic of oil since it is constantly being flushed away.
I continue polishing with the wet sandpaper until it is a better quality mirrored surface.
The "orange peel" that can be seen in the metal is caused by the grain structure of the crystalline formation of the steel. Further polishing would remove this effect, but it would take considerably more time. Mirrored chainplates are nice from an aesthetic point of view, but do not offer any additional corrosion resistance. Being how these chainplates are hidden away in a locker and in the head, the owner of this yacht was satisfied with this level of polish and fabrication continued on to the next step.
One important point to note, don't drill a single hole in the chainplate until at least this level of polish has been achieved. The sand paper used to get to this level would get torn up by all the edges of the holes. Sanding a solid and smooth surface is much easier on the abrasives.