I'm new to hand tools and have several old bench and block planes I'm refurbishing. One constant theme for me it trouble flattening the back of the iron all the way to the cutting edge. Unfortunately, none of my irons have had any kind of hollow back so it's taken a long time to get the back flat. I'm currently working on an angled block plane and I got the the first couple of inches flat using sandpaper on a granite block. Then when I got to my diamond stones (300/600/1200), i have a heck of a time getting it flat all the way to the edge. There's about a couple mm of area next to the edge where it's obvious that the diamond stone hasn't touched. I'm using plenty of downward pressure toward the edge of the blade - my fingers are fatiguing.
I tried Paul Sellers method of wacking the iron with a nylon hammer in an attempt to create a hollow back but that didn't work. I was afraid to hit it too hard, because I did that with a different iron and busted it. Any advice would be appreciate. The diamond stones are brand new and not the cheapo ones. I don't know if the swarf can get under the iron and cause this issue. I think I'm using plenty of glass cleaner.