I'm going to get a load of bolts and various bits of hardware zinc plated soon, but I was wondering if you're able to get parts with rubber or plastic bits in that can't be removed, plated? If so, what happens to those bits?
In my experience, many platers will render such parts unserviceable. Their prep finishes off parts that are already past their best.
The tailgate guides are a good example. The hard nylon blocks can fall apart/melt and the small springs which give them their action can be ruined by the chemical stripping and/or agitated cleaning process. They are all but impossible to take apart and rebuild. New ones are still available.
Same with the sprung lower guides for the door strikers. The hard plastic is easy to kill and the springs can break. Again, new ones are still available.
The door strikers themselves (the bigger part mounted to the door jambs) have rubber buffers inside them which are usually damaged or missing. When plating the door strikers (correct finish is 'clear' silver zinc) it is wise to replace these for correct closing of the doors. Rebuild kits, including the tiny rubber parts inside the door-side catches, are available.
Bonnet catch can be disassembled and the rubber buffer removed for plating. You can cut the rubber coating off of the finger plate and re-use it, or use something like Plasti-Dip to replicate it after plating.
Tailgate catches can be re-plated (correct finish is - again - 'clear' silver zinc) but there are springs in them which can get damaged. In my experience it is sometimes better to give them a thorough manual clean-up, rattle can paint job and oil-up. You don't see that much of them on the car anyway.
Platers can be a bit hit and miss. I've had to switch several times because of problems with lost parts, sloppy prep and plating and complete disregard for my written instructions (attached to the part!). One plater took it upon himself to chrome (chrome FFS!) a fuel filler cap because he thought "...
it would look better", even when it had a label requesting gold-passivated zinc.
I've tried the home plating kits but they are too much of a faff for me.