Phil, I presume you are talking about "card" scrapers? I don't know what you are using your scrapers for (because it can make a difference), but here is some advice from my own experience, from wich you can take what you will.
To add to what DW has said & maybe help you to grasp the principles better, let's start from scratch. First up, you don't need a 'burr' in some cases; a scraper can work with clean 90 deg corners, albeit not as well in most cases. When using a scraper in a holder (a combination known as a "scratch-stock") I rarely bother turning a burr, it's usually more bother than it's worth. However, for typical "flat" work a card scraper works infinitely better with a burr. Turning the burr serves two main purposes:
1) it work-hardens the edge. As DW has pointed out, scrapers are generally tempered to a much softer level than a plane blade, so a 'raw' edge on a scraper doesn't last long on tough woods. The work-hardened edge caused by raising the burr can be quite a bit more durable. (Edit: As DW has just said, "drawing" the edge [i.e. turning it back & forth a couple of times] increases the work-hardening & makes the edge a bit more durable still, but simply raising a burr is usually enough for practical purposes).
2). The burr acts to roll the shaving as it forms, in a similar way to a chip-breaker on a plane blade and the combination of this & the fine-ness of the shaving ensures you get very little tear out. Be aware, however, that a dull or coarsely-sharpened scraper
can tear fibres very effectively, especially on soft-ish woods (rule of thumb, the harder the wood, the better it scrapes!).
To knock off old paint or glue squeze-out, I generally use the scraper "off-file", i.e. no further honing after flattening the edge with a 10 inch smooth-cut mill file (the "mill" part is important, "mill files" are a much finer cut than ordinary flat files). Just turn an edge and away you go.
If you are levelling veneers or cleaning up a surface prior to finishing, you want a far more refined edge than a file leaves. To hone the edge of a scraper, you need
hard stones, something like India or hard "natural" stones or the scraper cuts & gouges & will make a right mess of your stone. You may be able to use one of the harder water stones if you have a light touch & are exceedingly careful, but I wouldn't attempt it myself. I use a fine diamond plate, but for many years before I had that I used a hard Arkansas stone. I hold the scraper very slightly askew to the direction I'm moving it, just enough to prevent gouging, but not too much or I have trouble keeping it vertical to achieve a perfectly square edge. Having a square edge gives you two good corners to turn, but if you have too much trouble achieving this, make one corner just a little more acute than 90* and use that. What won't work well is an obtuse corner, you'll get no workable burr on a 100*+ corner!
Finally, the burr itself should typically be just enough to feel with your finger. I've found that beginners nearly always roll
too much burr to begin with. About 3 or 4 passes with my (round) burnisher is usually plenty. If the burr is excessive, you'll find you have to lean the scraper too far forward to get it to cut, to the point where your little fingers are dragging on the wood, & they very quickly let you know they don't like that!
Some people get very passionate about what to use for a burnisher, but just about anything as hard as or harder than the scraper steel will work. When I started out 40+ years ago, the typical burnisher recommended was a screwdriver shaft. Screwdrivers are not the best for the job, they are tempered to about the same hardness as scraper steel so it generally takes more passes, the shaft also becomes worn & scratched & the resulting edge isn't as clean & straight as you'll achieve with a very hard material. Old solid-carbide bits are excellent, but the piece of 1/4" HSS I've used for many years does the job & shows not a single scratch or mark as a result.
People also have differening opinions about the ideal shape of a burnisher. After using a screwdriver for a couple of years, I ground the teeth off a 6" 3-cornered file & polished it up to 1200 grit. That worked much better, but at one time I was using curved scrapers quite a bit and found a round burnisher easier to use on curves. The smaller contact area means you get more "turn" for the same amount of pressure, which is why I say "just 3 or 4 light-ish passes" are usually sufficient. If you use a flat burnisher you have to press more heavily or make more passes to get the same amount of 'roll' on the edge because the force is distributed over a wider area. The round burnisher also gives me good "feel" of the edge and I can tell instantly if it isn't clean after honing or has become chipped from use (I typically re-turn the burr 3 or 4 times or even more between "sharpenings").
These are all things to think about when you start using card scrapers, but if you use them much, very soon it all becomes intuitive and you know just how much burr will work best for a given situation. I would say that learning to use a card scraper was one of the best lessons I ever learnt in woodworking. I live in the land of hard, gnarly woods, which scrapers take in their stride. Apart from the fine control & the quality of finish scrapers can give, they have saved me hours of sanding, for which both my arms & lungs are rather grateful......
Cheers,
Ian
