I have a whetstone that I was given, but it has very shallow dips in its surface; it is no longer level. Can I fix it, or should I just get a new one? is there a way to level a whetstone again?
it is no longer level.
It's worth mentioning right at the outset that a stone that is curved along its length is still usable as-is.
Honing stones/whetstones don't have to be flat to provide a suitable sharpening surface for all tasks. Certain honing or whetting jobs do require a flat surface (e.g. working on the unbeveled side of most woodworking tools, which generally need to remain dead flat) but honing the bevels of any tool, including knives, chisels and plane irons, can effectively be done on a curved surface.
This is precisely why many old stones are found profoundly hollowed or 'dished' in the centre in the first place — they continued to be used over time after becoming worn in the centre from use, and the hollow just becomes deeper and deeper over time.
should I just get a new one?
Assuming you can't use the stone as it is, totally up to you.
Be aware if you do want to flatten it the type of stone it is makes a massive difference in how easily flattening can be done.
Certain natural stones traditionally used with oil, and many artificial oilstones, are very hard indeed so they are resistant to abrasion to say the least. This is a great thing as far as use goes, because they tend to dish very very slowly in use (and if used carefully not really at all, even over many decades), but this is not your friend if you want to dress such a stone back to flat.
On the other hand certain natural stones traditionally used with water, and many artificial waterstones, are quite soft to very soft and these are very easy to re-flatten. Which is good as they need it often, sometimes daily, even with normal amounts of use.
Obviously in addition to the hardness or toughness of the stone the amount of dishing is a big factor:
The stone on the left here would probably require a great deal more work to put right than the one on the right!
So in summary, the labour necessary to get the stone back to flat might be considerable, or it might be not much at all.
is there a way to level a whetstone again?
Your stone sounds like it's dished on both faces but worth noting for future searchers that every stone has two faces and two sides, and it's very uncommon to see all three of the other long surfaces of a stone dished in addition to the arbitrary top face. Sometimes simply flipping a stone over can give the owner a flat surface with no effort expended! If both faces are dished as sometimes seen the long sides can be used if necessary.
If you do want or need to flatten you can use a wide range of abrasives, or abrasive surfaces, to dress or lap the surface of whetstones.
A non-inclusive list of things people have used include:
- the back of ceramic tiles
- the flat side of old grindstones
- a flattish spot on a concrete surface (pavement, driveway, even a wall)
- the side of a concrete block
- a brick
- another whetstone
- abrasive powder1 and water, usually on a glass or stone surface
- abrasive papers, films and cloths
- diamond plates
- diamond powder, suspension or paste on a metal plate
- belt sanders2
The process generally is just to rub the stone back and forth, sometimes in circles, until the high spots have been worn off and the stone has become flat again. Sometimes this is done dry, sometimes using water to flush away waste.
Obviously this goes much more quickly if the abrasive being used is very hard and coarse, so for this reason if your stone is a harder type and you have much material to remove I would recommend you use coarse silicon carbide powder (80 grit or coarser).
But that said you can do the work on concrete if you have to. I've done it myself a couple of times and while it takes a long time it does eventually get the job done.
1 Usually silicon carbide, but even sand can work albeit much more slowly.
2 Be extremely careful of the dust generated if you try to flatten any type of stone, but especially natural stones, with a belt sander. Some stones contain silica and fine silica particles are very hazardous to breathe in.
A whetstone can be leveled by lapping. This is essential grinding the grindstone and must be done with something harder than the stone itself (or perhaps as-hard in certain cases).
A very effective method, that I use myself, is to use a diamond lapping plate like this one:
With a setup like this, you can renew your bench stones many, many times, and have like-new performance for many years of frequent use. This treatment not only levels the surface, but exposes fresh grit for renewed cutting effectiveness.
But a lapping plate like this is somewhat expensive, so probably not the right option if you don't use stones frequently. There are other less expensive options like using loose silicon carbide grit on plate glass or a cast-iron lapping plate. These take longer and eventually the plate itself needs to be trued up because it is partially ground away itself during the process. If you search on "lapping bench stones" you'll find a lot of information on the other options.
The cheapest option is probably to buy a new stone, which you can get for about $7. I've had good luck with this "Bora" brand
If you do, you might want to pick up some mineral oil as well. It's cheap and is a very effective lubricant for sharpening on synthetic stones. The stone may come saturated with it; if not you should do that before using it to keep it from clogging up and losing its effectiveness sooner than necessary. After that, just use a little on the surface when sharpening, enough to keep the surface wet and float away the swarf and keep it from clogging the stone's pores, a condition known as glazing.
See this other recent question for a bit more: Can all whetstones be cleaned?
Just about any hard, stone-like surface that is fairly flat can generally be used to flatten a whetstone. This includes but is not limited to: other whetstones, bricks, pavers, concrete, flat rocks, sandpaper (or just sand) on a board.... Only your imagination will hold you back. Technique and paying attention to what you want to have happen are the key to success. The surface will need to be somewhere close to flat to begin with but both will naturally average toward flat with lapping.
There are many nuances for improving and tweaking but for most purposes, you can just rub two rocks together and get two flat rocks with the minor caveat that some of these options are better than others.
You might consider the conversion of a cheap whetstone (or expensive one if you're feeling lavish) to an efficient flattening stone by cutting a pattern of grooves across the width of the stone with a hacksaw.
If you decide it's worth the coin, the wonderfully effective diamond plates or purpose-made flattening stones are an excellent option for those suffering lower back pain due to chronic, persistent thickening of the wallet. Seriously though, not that bad a deal if you wear your stones out regularly enough and/or have a penchant for acquiring every old stone you find at a bargain.
It is highly recommended and likely necessary to use a liquid to carry away the detritus and keep the dust contained.
I have had success (with some variance in efficacy) using all of these materials (with the odd exception of the purpose-made stones but I feel comfortable assuming they work great) on newly acquired old stones. However, once I had acquired a couple, I mostly just use other whetstones because of their hardness and wear characteristics as well as the efficiency of flattening two stones at a go.