[Alert! boring description of Scary Sharp - ignore this if you sharpen in other ways!]
On Record crucible cast steel: I've got three irons of that now, and have a brilliant edge on them all.
My No7 is a 'bitsa': C19 Stanley body with rosewood handles, but a Record blade, chipbreaker and a Stanley cap iron. I bought it for the body, but when I got it home I found someone had completely stuffed the iron. In an attempt to re-grind it they'd ground it square with no bevel at all, then given up!!!
The first time I tried it to grind it, I was daft enough to play with the 'stone grader' on my Dakota wet grinder ("Tormekkalike"). It's a silly idea, and of course it doesn't actually work (your mileage, etc.). Like you, I got a burr and couldn't get an edge (it sort-of worked with later Stanley bog-standard irons).
Over the last couple of days, I've been through the set, sorting out any dodgy edges (currently 13 planes - slope, what slooooooo... ow!), and I had another go at the crucible cast irons. This time I did it properly and got wonderful results.
Here's how it works (consistently) for me:
0. polish the back to a mirror finish, using the scary-sharp papers (see below).
1. you really need a slow-speed or well-cooled grinder. Grind the basic 25degree bevel.
2. use "scary-sharp" to sharpen, with an Eclipse-type guides.
it's fast and easy to get good, consistent results:
3. Apply a Charlesworth micro-bevel. This is staggeringly useful, but mainly for bevel-down planes to increase the effective bevel angle. David uses a ruler, but I just use another plane iron:
When scary-sharp is done, put the iron you're sharpening flat side down (bevel up) on the 2500 grit. Prop the back end up on the side of another plane iron - just the thickness of it is enough to get the microbevel. Make a few small circular movements lightly on the paper, and you're done.
#3 will make a surprising difference to the crucible cast steel, mainly because it's old. Any rust pitting is quite hard to remove from the flat back without taking too much metal away, but the micro-bevel cuts back past it, giving you a sharp, well-defined chisel-like edge.
Not having a mirror-finish back will prevent you getting a really good edge, rectified to an extent by a micro-bevel. You only need to flatten once in the lifetime of the blade, as long as you keep it clean and rust-free.
Normally, when doing scary-sharp, I stick parallel strips of wet+dry to a piece of thick glass. When flattening, I start off several grades coarser than normal (160 or 180 grit for this), as it would quickly clog otherwise, and I use one strip at a time stuck along one edge of the glass plate. That way the plane iron is made easy to hold by putting that edge of the glass along the edge of the bench. Always use the paper wet, too.
Tip: use an old 50mm camera lens back-to-front as a very high-quality magnifying glass. When doing scary sharp, each step up the grits is done when there's a new, even set of 'grinding' marks, obliterating the coarser marks from the previous step. The change is easily seen. With practice you can feel when you have achieved this and you won't need to look.
The finest grit I use is 2500, but I believe you can get incredibly fine 3M stuff from Workshop Heaven for Samurai-grade sharpness.
. . .
How can you spot a left-handed, traditional woodworker?
Their right forearm is bald.
I wish I could sharpen freehand, but I don't have the muscle control any more (nerves are shot, annoyingly).
Scary sharp with an Eclipse guide is quick and effective for me, and I'd recommend it to anyone.
Hope some of that helps,
E.
