Generally speaking, you'll want to use coarse grits (P8 - P80) for stock removal and the stripping of paints and varnishes. You'll then want to use medium grits (P100 - P220) for prepping and shaping materials followed by fine grits (P240+) for sharpening and polishing.
To start, choose one that's just coarse enough to easily remove imperfections. If your surface is already fairly clean and smooth, choose a medium-fine grit like #120, progress to #150, and end with #220. For rougher, lower-grade wood, start with #80, progress to #120, and end with #220.
Choosing the Right Sandpaper Grit
For heavy sanding and stripping, you need coarse sandpaper grit measuring 40- to 60-grit; for smoothing surfaces and removing small imperfections, choose medium sandpaper from 80- to 120-grit sandpaper. To finish surfaces smoothly, use a super fine sandpaper with 400- to 600-grit.
Because of this, the sanding process usually involves starting with lower grits and advancing to higher grits. Each increasing grit number will smooth the scratches left by the former grit, resulting in a surface that's smooth to the touch and without visible scratches.
You don't need to go past 180. Grits higher than that are for cleaning and polishing between coats of finish. It you go past 220 you may even encounter issues with the finish not adhering properly.
There are instances where you can use 180 to 220-grit to rough up a piece in preparation for paint, but you do not typically use these grits until after your first coat.
Oversanding also occurs when you do not stop after a certain fine grit and keep on sanding with even finer grits to achieve a smoother finish. This often happens when you sand beyond 180 grit level which leads to excess sanding and removal of too much wood material.
The Golden Rule of Sanding
The golden rule for choosing your sequence is to never skip more than 1 grit. For example, if you start with P80, and need to finish at P240, rather than using every grit from P80 – P220, you can do P80 – P120 – P180 – P240. This sequence cuts out P100, P150 and P220.
If your knife is simply dull and doesn't require repair, like fixing a chipped tip or nicked blade, a good starting point is between 800 and 1,000 grit.
With enough time and energy spent sanding, you can still end up with the same result. However, because we skipped so many grits, it is going to take much more time and effort to lower the 80-grit peaks and establish the 180-grit scratch pattern that we need for a high-level finish.
Fine Sandpaper, ranging from 120- to 220-grit, is used in most home workshops. This sandpaper is often used for final sanding before a project is completed. Extra-fine Sandpaper, including paper with 240-grit to 600-grit, is used between paintings or varnishing and is best suited for polishing jobs.
The material you're painting will also determine if sanding between coats is necessary. As a general rule, if the surface you just painted doesn't feel or look smooth, sand it!
Fine Grade = 240 grit. Perfect for preparation of the natural nail. Used for light refining of product. Made of the same abrasive paper as Willow Nail files.
Make sure to sand the polyurethane between coats.
Sand Your Workpiece
Always start with a coarse grit belt or disc and work your way progressively through finer and finer grits until you reach the desired level of smoothness.
To begin, select a stone in the middle of your grit options. This may be a 1000 Grit water stone, a fine DMT Diamond Stone, or a fine Norton India Stone. This really is guesswork at first, but with experience, you'll get better at it. Using your chosen grit, give your knife a dozen or so strokes on one side.
If your whetstone has a coarse and fine side, begin with the coarse side. If you use two stones of different grit, start with the one with the lower grit number. Hold the knife with the handle in your dominant hand.
The standard number of a medium stone is around 1000 grit. This is the standard whetstone for sharpening, and is the first kind of whetstone you should buy. Basically, they are used to fix dull knives and restore cutting ability to a knife's blade.
Generally speaking, you'll want to use coarse grits (P8 - P80) for stock removal and the stripping of paints and varnishes. You'll then want to use medium grits (P100 - P220) for prepping and shaping materials followed by fine grits (P240+) for sharpening and polishing.
One common mistake is not replacing the sandpaper frequently enough, especially when it becomes clogged. This often happens due to a lack of experience or the desire to save money by using fewer sandpaper sheets. However, continuing to sand with worn or clogged sandpaper is a big mistake.
If you need to use a sequence of grits for your workpiece, it is important to remember one thing. That one thing is Never Skip More Than One Grit.
To know when you are done sanding, look at the wood in a low-angle reflected light. Or wet the wood then look at it from different angles. Before you apply your finish, raise the grain by wiping the surface with water. Then sand lightly to remove the nubs from the raised grain.
Sanding sealer should be used after the wood has been sanded down to a bare finish but before the topcoat (such as paint) has been applied. Below are some uses for sanding sealer: Painting medium-density fiberboard (MDF)
Isolated sanding marks can be addressed by repairing/resanding individual areas. Resanding the affected areas may require the first cut with the big machine to be on a steep angle, followed up with straight cuts. Use of a hard plate, multi-disc or planetary sander also will help alleviate some sanding marks.