I've had mixed luck with Solder-it. It is finnicky because pot metal is difficult to solder in the first place. Getting it to stick is one thing. Getting it to flow deep into the pits is something else.
There is also a product called MuggyWeld that works pretty well. But again, it is not easy to use and is available only in stick form.
http://www.muggyweld.com/potmetal.html
I prefer to flash copper and then use regular lead-based solder paste to fill pits. If the pits are deep, build up with regular stick solder after using the paste.
Heat only to where the solder flows in. Don't cook the piece; you might delaminate the copper...or destroy the pot metal!
Wet sand the lead solder and catch the solder filings in a small tub or pan. Don't wash it down the drain!! Let it settle, pour off the water, and "mop up" the tub or pan with paper towels and dispose of the towels properly. Sanding lead dry is one of the worst things you can do for your health and the health of your family (especially kids!!). Wear gloves.
Don't even try Pot metal without sandblasting. If you don't have a sandblaster, you can get a very serviceable unit for around $100 that will serve you well for years.
Ken