I have some router-bits that are getting dull and need to be sharpened. I have 220, 400, and 600 grit sandpaper that I could use to give a sharp edge (if possible). If I need to get higher grit sandpaper I am cool with that. I know you can do HSS (High Speed Steel) but I would like to know for carbide.
If I need to get a diamond-tipped file for sharpening I will but if I can use the sandpaper that would be neat.
Can I sharpen a carbide tool with sandpaper?
Without knowing exactly what kind of paper you're referring to the answer would have to be maybe.
The term sandpaper is used very loosely, in addition to the many different grit types and bonding styles in various papers it can even be used by some to cover sanding materials that aren't paper-backed.
Some woodworking sandpaper will basically have no effect on carbide, it's much much too hard compared to the abrasive grit on the paper (quartz or flint). You can test this on the shaft of a solid-carbide burr, where you won't be able to get the paper to leave any scratches.
You can get some traction with aluminium oxide or silicone carbide papers, although the going will be slow. These two abrasives are used in most high-end abrasive papers, cloths and films (including wet-and-dry types) and they can be used to polish carbide. If they can polish it they are 'cutting' the material so can be used for sharpening too.
But in general this is something that calls for diamond plates of one sort or another:
You do have to be very careful about not altering the geometry of the bits and maintaining symmetry, so some more info:
Q & A: Can I Sharpen My Router Bit? on Popular Woodworking
Sharpening Router Bits on Rockler
Care and Sharpening of Router Bits on Highland Woodworking
A cheaper alternative to a commercial diamond plate would be to use a diamond paste applied to a flat steel plate of some kind. Diamond pastes are an inexpensive entrée to using diamonds in sharpening, particularly if you buy them directly from China. They are available in a wide range of grit sizes — commonly from 0.5 micron at the fine end to 30 or 40 micron at the coarse end (still relatively fine by abrasives standards) — and can be suspended in oil or a water. I would recommend going with the oil-based type for most users.
Note: the dust from carbide is considered highly toxic so it's advisable to take appropriate precautions to protect yourself from inhaling any as you're sharpening the bits and wash the sharpening media down well afterwards if not using an oily medium.
(Unless you buy Silicon Carbide (SiC) sandpaper)
Every "ordinary" sandpaper is made using either Silicon Dioxide (SiO2) or (more expensive) Aluminum Oxide (Al2O3).
The Mohs scale is one way of assessing hardness, its principle is "harder materials scratch softer ones, not vice versa". Harder materials have higher numbers on the Mohs scale.
Mohs hardness is about 9.5 for Tungston Carbide (WC) (give or take 0.05 depending on composition), about 9.0 for Al2O3, and 7.0 for SiO2. Which means neither of the usual sandpapers are able to work as abrasive for WC.
You need something with a hardness above 9.5 for being able to scratch/abrase WC, which only leaves SiC (9.6) and diamond (10.0), or Boron Nitride (BN) (not available as sandpaper, more expensive than gold, and poisonous).
Which means that, regardless of what anyone says or what someone shows in a youtube video, for any kind of "ordinary" sandpaper, there is no way you could use it to sharpen a WC tool.
They are either cheating (using SiC and not telling, or not using a WC tool) or telling fairy tales. Or maybe their sharpening is placebo effect.
SiC sandpaper costs about twice as much as Al2O3 here, and the price of diamond sandpaper is decidedly in the two-digit currency per sheet range (so not really a practical option). Your mileage might vary.
SiC is just about hard enough for WC, but don't expect it to last long.
