Jim Patrick's thread in regard to nickel plating leads me to a question that I have wondered about for a long time:
Many years ago, when I was a kid in suburban Chicago, my first car was a '28 Model A Standard Coupe. (No,...it wasn't new,....I'm old, but not THAT old!) I had an elderly friend at that time that I'm sure is long gone now, but he had a small chrome plating shop in Chicago, and most of his business was little jobs that most of the big chrome plating shops would not bother with. For example, he did a lot of plating of musical instruments for musicians and orchestras. As my friend, he did a lot of chrome plating for me very cheap, or, being the nice old fellow he was, sometimes for free. He also told me a lot about the plating business at that time, most of which I've forgotten. I do remember that it's basically, "polish, then copper, then polish, then nickel for color and then more polish, then chrome last. One thing he felt strongly about however, has always stuck in my mind, and I thought it would be good to ask those on this forum that have a much greater knowledge about chrome/nickel plating than me:
My old friend, Ray Thompson, was very, VERY emphatic that whatever he plated for me, especially thin stamped things like the radiator shell for example, MUST be painted on the back side. Ray explained that, strange as it may seem, metal is somewhat porus, and that even tho' it's a very slow process that takes many years, moisture will work it's way into the thin sheet metal and EVENTUALLY attact the plating from the back side. He was so convinced of that that he made me promise to paint the back side of the radiator shell with rustoleum before he'd rechrome it for me. (I know, it was supposed to be nickel, but his main business was chrome).
Anyway, I've always wondered if this "moisture seeping thru' the backside over a period of years and attacking the plating from inside" was just an unsubstantiated "theory" of an old man, or if there is some truth to what he said. Obviously, he certainly knew his business and certainly believed it.
Anybody know anything about this? Does it really make sense to paint/seal the back side of chrome or nickel plated thin sheet metal pieces? Anybody,.........?
I do small lot plating but I never heard of that. What does happen though, depending on the process and anode setup, is the backside may have little or no plating. The thin/porous plating won't stand up so the metal starts to rust. The rust will go through thin metal to the plated side and show as bubbles until the plating flakes off.
I've used lacquer paint as a mask in electroless nickel plating. Parts like a starter or generator housing don't need the inside plated. The mask reduces load and can be washed off easily with thinner. Without the mask, the entire surface area would be plated. If using an anode process, areas not in "line of sight" of the anode(s) don't get plated. This is the shadow effect and may be the reason your friend wanted you to paint the backside as well as giving you a better price.
I always painted the back side if it does not show just because you never clean that area and we all know if you don't clean the part and wax it once in a while, you will get rust and pits over time. It can't hurt anything but I never paint it till its been plated first.