It is probably the most difficult (but not impossible) metal to plate.
If you have any hair....take it out now!
To start with you have a zinc based metal that was of no specific alloy or make up. Then add to that about 40 or 50 years of corrosion and it gets frustrating.
But...as I said, it can be done.
The way I deal with it......and there can be other ways. is to first
Sandblast the part really, really good.
This does three things...
1 Removes the chrome. (you will see it spark as it comes off)
2 Etches the nickel and gives the strike plate a good foot to plate onto.
3 Exposes and removes some of the corrosion from the pits.
Take your part out and blow it off with air.
You most likely have a part that looks like it belongs up in space floating around now.
Next I clean off about 2 square inches of the surface with a wire wheel in a 1/4 die grinder. Make it all shiny.
Then take a drill bit or dremal tool and drill out each pit down past that little black dot of corrosion thats in the middle of them.
Now for the fun part.
All those little pits have to be filled and smoothed out.
I use a zinc based solder and flux that melts at 340 F. Its tricky to use but works well.
Fill each pit, wash and sand smooth. Repeat this until the part is totally smooth.
I sandblast the part again after this. Just lightly.
Dont touch the part with your bare hands after this. Use nitril gloves..not latex or rubber.. They will leave a residue on the part
I now put on my robbers to protect the High current areas. Put the wire or heavy piece of copper on the part that will hold it to the cathode bar.
Make a little jumper wire that goes from the cathode bar to the part. This will allow you to enter the STRIKE bath live.
You want to go into the strike bath live and at the highest amperage setting you can without burning the part. This does two things.
1 Gets the plating on as fast as it can so the bath doesnt react to the zinc based metal
2 Makes the bath throw down into the Low current areas better.
After about a minute like this take the amperage back down to what you figured out for the part.
This will take some practice on some SCRAP pieces.
After you are satisfied with about 15 miniutes of the strike bath rinse your part off and go directly to the acid copper for at least a hour.
I usually go 90 minutes on the first acid copper plate.
After the acid copper, rinse the part off and start sanding it smooth.
DONT sand down to the original potmetal. If you do, continue to sand the rest of the part, rinse off with plain water and into the strike bath to restrike it.
Use gloves when your sanding so you dont get the copper all over your hands and you dont get your finger oil on the part.
Continue to sand and copper until it is smooth. I sometimes have to copper plate 5 times to get all the pits removed.
Once it is smooth I then work up from 180 grit to 400 grit sand paper and then buff, clean, nickel plate, rinse and then chrome plate.
Like I said.....PRACTICE on scrap pieces...dont take your good parts and ruin them.
I hope this helps shorten the learning curve for some of you.
Pot metal is a mutha......but it can be plated successfully by the hobbiest plater. It just takes time and patience...........lots of patience!
Now you know why the plater groans when you bring potmetal to them!