How do both techniques compare? In terms of result, cost, applicability to different kinds of wood, different shapes of work pieces and cleanliness of process.
I always used sandpaper to get a smooth surface, which you have to buy again and again. Getting one scraper that does the same (does it?) sure sounds tempting to me. I wonder if I could substitute sanding with scraping, either entirely or partially.
How do both techniques compare? In terms of result, cost, applicability to different kinds of wood, different shapes of work pieces and cleanliness of process.
Good question.
As a preamble to an in-depth comparison between scraping and sanding let me frame my answer in terms of 'surface preparation' which is what you are doing when you are either scraping or abrading or both.
The goal is to prepare the surface of the wood or veneer for a final finish, or in the case of unfinished wood pieces (shoji comes to mind) to achieve a final surface before use.
The benefits of one method vs another are best defined in terms of what type of finishing you are going to pursue and any cost or time restraints which your project may have (an important consideration).
Bear in mind you are omitting planning, which is another very viable method of surface preparation, but I will limit this answer to the two methods that you have mentioned.
Abrasives are pretty freaking awesome. But they are a fairly new method of preparing wood surfaces in the long history of working wood. Nowadays however, you will be hard pressed to find a hobbyist or professional who has not spent their share of time 'sanding'.
Abrasives excel in a few areas that scrapers cannot begin to compete with.
The first of which is time. If you are preparing case-goods, cabinetry or solid wood surfaces then you will most likely gravitate towards an electric or pneumatic sander.
Abrasives can achieve very consistent surfaces that will allow your finishing process to be quite predictable. If you properly work through the girts on your way to your final surface you can have a very well prepared surface indeed, with a little practice.
Versatility is another area where abrasives can really shine. If you are making a custom moulding or preparing a concave or convex surface, sanding with a custom backing pad or by hand is almost the only way to go unless you want to make custom scraper blade profiles.
Ease of use. I think this one speaks for itself.
Abrasives have several drawbacks that you are probably already aware of:
Abrasives produce both wood and abrasive media dust which is considered an occupational hazard of woodworking. This is kind of a big deal if you are a professional. Even for the hobbyist, you will want to take measures to limit your exposure to sanding dust by using a good quality respirator of some kind and ventilating your shop. Even afterwards, this dust can irritate your lungs when you are long done with sanding and so regular cleaning is also required.
Abrasives are expensive. I mean they won't break the bank, but you will be spending money on sandpaper for sure. And like most things you get what you pay for.
Flatness. If you are looking for a superiorly flat surface for whatever reason, abrasives may let you down. Due to variations in wood hardness, sandpaper tends to inconsistently remove material from most wood types. An extreme example is what is known as 'heart pine'. The difference in hardness of the early and late wood of the annual rings is tremendous. Sanding this wood improperly will produce a very undesireable result without care.
Once the method of choice for wood surface prep, now the scraper has been relegated to the tool cabinet for most applications.
Flatness. A properly burnished scraper can easily produce a high degree of flatness in a surface. But lets expand this to a curved scraper and you will find that a scraper can hold a very 'true' profile as well - good for custom moldings. But don't necessarily get the idea that the scraper is exceptionally good at flatness, as a hand plane will beat the scraper every time.
Veneer and burls. Where a hand plane would be a totally bad option, before abrasives came into vogue a scraper was the only other method. It is still a very viable way to quickly bring a freshly veneered surface (don't think plywood as this has already been commercially sanded) into the neighborhood of ready for finish. Scrapers are easier to control when preparing veneers and will let you avoid 'sanding through' which is all to easy to do with a power sander of any variety.
Ease. Think of this strength of the scraper as a byproduct of not having to sand a surface through 4 consecutive grits. A good scraping and most hardwoods no longer have tool marks and may need only a light sanding (320+) to be ready to sand. If you are removing planner marks with a power sander you probably need to start at at least 180 grit and work your way up.
Getting a scraper ready to use is a bit of a trick. But don't let this fact cause you to think that using abrasives involves less skill. I have seen far more pieces of furniture that were improperly sanded.
As mentioned in the comments above, scrapers can be very hard on your hands when used vigorously. I have actually burned my thumbs with a scraper blade due to the heat of the friction of use. Not to mention that it does take considerable stamina to scrape anything of size.
As the scope of a piece of furniture grows, you probably need another tool to do your surface prep. A solid wood dining table should probably be planned or power sanded as you will wear yourself out scraping that bad boy. And if you scraped everything you could as a professional wood worker you just might starve to death given the time it would take.
This is a very good topic to have here since there is active debate going on in woodworking circles about the overuse of sanding to smooth wood in the modern era.
How do both techniques compare? In terms of result, cost, applicability to different kinds of wood, different shapes of work pieces and cleanliness of process.
Where possible scraping is always preferable. That's a strong statement but it's easily backed up.
Results Using conventional scrapers and when done properly scraping is capable of creating a surface ready for finish, so that's equivalent to roughly 180-240 grit paper. In general the smaller the burr on the scraper the finer the surface it will create.
Using an unconventional scraper, one with a sharp knife edge, you can get to a surface even finer than this, equivalent to 600 grit if not higher. However, this is not needed in almost all cases and can be counter-productive as a wood surface that smooth can resist finish. On very hard, resinous hardwoods though knife-edge scraping can create a superb surface, requiring nothing more than hard buffing to create a shine.
But talking about the smooth surface scrapers create is only the tip of the iceberg, since you can go from sawn lumber directly to a finish-ready surface using just the one tool, rather than working up through the grits as we have to using abrasives - making a card scraper literally the equivalent of sandpaper from 60 grit through to 220.
Cost There is simply no comparison on cost. As I like to put it, a scraper is like an infinite stack of sandpaper, which is a neat trick for something that costs perhaps 10 USD and at best can be free.
The costs associated with sandpaper build and build over time because sandpaper is a consumable resource (and for best results users should be encouraged to use it and discard it more frequently), but card scrapers are lifetime tools. Several lifetimes in fact; if cared for your grandchildren could be using the same card scraper you bought or made for yourself last week.
Applicability to different kinds of wood There's some contention here. Conventional thinking is that you can't scrape softwoods but as Chris Schwarz has written about and as I too found out for myself, you can.
Scraping can achieve good results in pretty much any wood in good condition, most certainly hardwoods. Spalted hardwoods could prose a problem as areas within the spalting can be soft and 'punky' which means they are too soft to scrape successfully.
Cleanliness of process This can be summarised by: shavings, not dust.
Other advantages
Faster Despite being a hand process compared to a powered process, it is actually faster to go from rough wood to finish-ready.
Quieter Obviously there's no comparison between a hand process like scraping and power sanding, which is a key benefit for those woodworking in the home (your family members and neighbours will thank you. Plus no more ear defenders or ear plugs for you.
Never clogs Unlike sandpaper a scraper can't clog. Resinous woods can build up a residue behind the edge, but this is easily scraped or wiped away in a moment.
Works on finish as well as wood With a light touch scrapers are great for smoothing minor imperfections in finish, but are actually amazingly good at removing finish entirely if needed.
Disadvanteges
It couldn't all be upside. Scraping is a hand process, so by definition requires muscle effort. On large projects in particular scraping can be extremely tiring, not just on the arms but on the small muscles in the hand in particular because of the conventional hand grip seen in the above image. This can be offset somewhat by using a commercial scraper plane or holder, or a homemade wooden equivalent:
I wonder if I could substitute sanding with scraping, either entirely or partially.
I am a big proponent of scraping and use it as much as I can to help keep dust from woodworking at a minimum, however it cannot completely take the place of abrasive papers/films/cloths. Sometimes a quick swipe with a piece of 150 paper is all that's needed so there's no reason to religiously scrape in every instance.
Traditional versus heretical scraper sharpening
The traditional way to 'sharpen' a card scraper is summarised as follows: file the edge, hone the edge against a stone to remove file marks, hone both flat faces to remove burr from filing, prepare corners for burr formation by burnishing flat ("drawing the steel"), lastly turning the burr using the burnisher. The process is not particularly long or involved but some users experience difficulty in repeatably achieving good burrs along the whole width of the scraper. The entire process is not needed every time, a burr can be re-turned a few times before the edge must be re-prepped from scratch.
There is a faster modern alternative however: skipping every step but the first one.
After filing there is already a burr along each edge of the scraper, which is of course exactly what the sharpening process seeks to create. They are evidently not as refined as those created by the conventional preparation method, but they are capable and by using finer files the burr can be made smaller and less crude than those created by a medium or bastard mill file.
By using the card scraper directly after filing the preparation process is cut from minutes literally to seconds, taking no more than 10-20 seconds to do. Because of the speed and ease the user is more likely to sharpen as needed, rather than put off sharpening past the point where scraping results are not as good as they could be.
Note: in addition to card scrapers there are cabinet scrapers, which are not exactly the same. Typically the blades are thicker and the edge is prepared differently (with a 45° bevel and one pronounced burr at the apex) and they are almost always used in a scraper plane of some kind.
Homemade or adapted card scrapers
Scrapers traditionally were made from old saw blades that had broken or bent and couldn't be made serviceable again. This is still a viable way of making your own scraper. The blades can be cut down using a motor tool with a cutting disk or slitting disk, or as conventionally by sawing through with a hacksaw (tip: grip the saw plate with scrap wood on both sides to prevent
If you would like to experiment with knife-edged scrapers, boxcutter blades and single-edged razorblades make excellent small scrapers, perfect for board edges, getting into tight corners and for fine surface touch-ups. They're also excellent for smoothing minor imperfections between coats of finish. Rounding or "knocking off" the corners slightly can be advisable to help prevent leaving track marks in the work.
Sharpening these blades is possible, but tricky to do well (very shallow bevel angle) so it may be necessary to use one until it no longer scrapes well and then discard.
For larger surfaces a kitchen knife can be pressed into service, even inexpensive stainless knives have steel that is up to the task. These can be honed or re-sharpened as needed by any conventional means, I use a standard rod-shaped knife steel and steel the edge as soon as scraping efficiency goes down and scrapings are no longer being produced.
Scraping is what they did before there was sandpaper. Scraping yields a different texture surface that feels more like wood, due to the fact that it is shearing the wood fibers rather than grinding them away.
Sand paper is very popular because most people (myself included) haven't bothered to learn how to use card and cabinet scrapers to their fullest. Since you are the one that sharpens a scraper, you can determine how aggressive the cut will be.
Some people (myself) use scrapers primarily to clean up glue joints. If you want to use a scraper, it is important to learn how to properly sharpen and burnish them for the specific uses you want.
Sanding abrades the surface, leaving scratches everywhere. Scraping actually cuts the wood fibers, leaving a sharper contrast of the grain in porous woods like oak and walnut. On more even grained wood like maple the difference is a lot less noticeable, provided the sanding was accomplished correctly.
As for results, it depends on the final finish. For a fully pore filled finish, virtually no difference. For a thicker film finishes, 2-3 coats of poly, little to no difference. For a close to the grain "oiled" finish with little film thickness, it is noticeable, especially if the sanding is not done correctly. More porous woods will show more difference.
Another aspect is surface flatness. A scraper can easily dish a surface. A proper sanding block or powered sanders won't (used correctly). A Stanley #80 cabinet scraper is much better for flattening a surface than sanding or hand scraping, and won't dish a flat surface. For the hobbyist, the way to flatten a surface is a hand plane, and then a cabinet scraper or scraper plane (or very good smoother plane) is used to clean up tooling marks.
Project shape plays a major role. One would need a lot of different scraper shapes. These can be purchased or cut from rectangular card scrapers - some use broken glass. This aspect starts to drive up the cost of scraping. I have many different scrapers, from rectangles to various curved flat shapes and others that attach to handles. A lot depends on whether the scraping is for finish removal or surface prep. There is cost associated with the chosen sharpening/honing method and a burnishing tool - carbide is preferred and does a better job, but hardened steel can work.
Cleanliness: subjective. My ROS attached to the shop vac captures the majority of the dust. Scraper shavings are cleaner and safer than hand sanding dust.
Ergonomics: holding scrapers by hand is much worse than hand sanding or an ROS. There are holders for rectangular scrapers that drastically improve the ergonomics, and then there are cabinet scrapers that are a real improvement.
My journey started with wanting large flat surfaces, which after trying sanding and router sleds led to hand planes and then scrapers, cabinet scrapers, and scraper planes, which reduced my use of sandpaper to very little. I typically only use 320 grit and up now, depending on the project. My advice is to start with a #80 cabinet scraper and see how you like it.
