- The Basics of Grit
- What grits should I use?
- The Golden Rule of Sanding
- Why You Should Follow The Golden Rule Of Sanding (or how sanding works)
- How to choose your grits
- Applications and Tips:
Whether you’re new to woodworking – and therefore, sanding – or you’ve been around for years, choosing the right grit sequence is key for achieving a beautiful, even and smooth finish.
We know as well as anyone that sanding through the grits can be a tedious affair, and we’d all rather be doing something else. But, when we cut corners, it definitely shows in the finish… Why? Well, we’ll get to that. But, first, let’s discuss the basics of grit.
The Basics of Grit
Sandpaper, or coated abrasives, as the broader category is known, are composed of abrasive grains, adhesive and a substrate backing.
Coated abrasives, such as sanding belts, discs or sheets, come with a variety of standard grain sizes, more commonly known as grit, or grit size. The sizes are standardized and determined by two main methods: sieve number or micro grain. With the sieve number, grain size is determined by the number of meshes per inch, whereas with micro grains, the size is determined by the sinking speed during sedimentation.
There are different systems out there, and typically, Uneeda’s products follow the FEPA standard, in which grit is denoted by the letter P and then a number (e.g. P100), where coarser, or larger grains, have a lower number and finer, or smaller grains, have a higher number. Sometimes a # sign is used. Depending on the material, we offer products from P24 all the way to 10,000.
This is much more easily explained in the following graphic:
What grits should I use?
You might be wondering, “does that mean I need to stock all the grits, from 24-2500 at all times?”
Well, no, not necessarily. What grits you’ll need, and in what format of product (belt, disc, sheet, etc.) will depend greatly on your sanding application. For instance, if you’re only ever going to do rough sanding, vs if you plan on only using stain finish, or if you need to refinish cabinets or sand lacquer or other shiny finishes, will all influence what grits you should have on hand.
If you don’t need to sand at every stage, you can limit what grits you need… but, even if you do need to sand course through fine, as long as you stick to the golden rule of sanding, you still can get by without keeping every grit available in the shop.
The Golden Rule of Sanding
So, what to do if you need to rough sand all the way on through to the finer grits for finishing?
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.
Caveat: This rule can generally be applied with higher quality abrasives, and we have seen it hold true with our own products. We can, however, make no guarantees for products from other manufacturers. The only way to be sure is to test it out in your specific situation.
Why You Should Follow The Golden Rule Of Sanding
This comes back to how sandpaper works.
When you are sanding, you are applying a sharp cutting tool to your surface and creating scratches made up of peaks and valleys – known as RA and RZ. When you use a coarse grit, you are creating a deeper scratch with higher peaks. The goal is, throughout your grit sequence, to even out the peaks and valleys so that the surface will become smoother, without “invading” the valley. If the valley of the scratch is invaded, the wood will close, causing pooling and a blotchy finish.
Therefore, each abrasive must be able to remove the scratch from the previous grit, so, for example, if a P180 follows a P80, the P180 will not totally remove the scratch. This will lead to over-sanding and swirls in the wood.
When you do follow the proper technique and Golden Rule of sanding, each successive grit will remove the proper amount, leading you to a more even scratch pattern on the final pass.
In the illustration, you can see the highest peaks are the lowest grit, say 80, and if you skipped to 180, it only cuts the top, leading to a very uneven scratch pattern, versus the pattern all the way to the right, which is very even, and happens when you progress through the grits without skipping.
How to choose your grits
When evaluating the application to determine your grit sequence, start by asking yourself the following questions:
- What stage am I at in the sanding process – calibration, leveling, shaping, preparation for finish, between coats of finish, scuff sanding, fine sanding or polishing – and what am I trying to accomplish?
- How much material do I need to remove on one side of the work piece?
- How many sanding units do I have available for the total sanding process (more applicable to wide belt sanding)
- What tool(s) will I be using, or will it be hand sanding?
- Which grit do I want to finish with?
- What material am I sanding, and is it hard or soft? Is it very resinous, like pine?
These questions can give you a good sense of where to start.
When determining your grit sequence, a good rule of thumb is to choose the finest starting grit possible to achieve your desired removal rate. This is because, while you could start with a lower grit, it will not only create extra work to compensate, but also will be a less efficient sanding process.
Here is an example of how you might follow use this as a guide yourself.
You would like to sand solid beech. The total removal on one side is 0,8mm. You have two sanding machines with two units in each. The required finish of the surface is P220.
P60 + P100 on the first machine and P150 + P220 on the second machine is most efficient from the point of lifetime of the belt. If you prefer little bit better finish, use P80 + P120 and P180 + P220 instead.
Applications and Tips
Following are some additional applications and various tips and guidelines to get you started.
Applications by Grit Number
- Remove bumps in the wood
- Remove old paints and finishes
- Rough lumber
- Starting grit for hardwoods
- Remove shallow scratches
- Remove saw blade burns
- Starting grit for softwoods
- Prep for polyurethane & enamel paints
- Raised wood grain fibers
- Surface irregularities
- Prep for latex & acrylics
- Begin surface finishing
- Prep for wood stain
- Sanding between coats
- Wet sanding
- Final surface finishing grit
- Smoothing top coats
- Prep for finishing oils
- Prep for polishing
- Metals and plastics
- Fine wet sanding
Tips by Material/Application
- The first belt you use should be P100 or coarser
- The first unit in the machine should be a metal drum or hard contact roller, with hardness measuring 80 – 90 durometer
- Only jump over one grit e.g. P100 to P150, skipping P120
- Do not remove too much with the last belts (to avoid wavy surface)
- Do not use finer grit than P240 for the last belt (in general enough)
- The first belt you use should be P120/150
- Remove 0,15 -0,2mm to obtain best surface
- For white/raw wood, do not use finer grit than P240 for the last belt, as going finer may cause issues with the wood accepting stain.
MDF for painting
- Use sanding belt with grains of silicon carbide. Important on the last sanding belt
- Use grit P240/280 on the last belt for best finish
The above image features a test board showing the absorption rate of stain with different final sanding grits, ranging from 80-320, from left to right. The entire board was sanded at 80, then the following sections followed the Golden Rule. (80; 80-120; 80-120-180; and 80-120-180-240-320)
This test is not an indication that you should choose a particular ending grit, but rather an illustration that there is more absorption at lower grits. The best ending grit will be according to personal taste, so it is recommended to do a test board with your actual material and the stain you plan to use. Also, check on any guidelines provided by the manufacturer of the stain product you plan to use.
- Do not use too high sanding pressure and belt speed
- Use a soft pad or contact roller
- Check that the graphite is not damaged
- When sanding hard lacquers a product with grains of aluminum oxide is recommended because this grain type will cut the hard surface more easily.
As you can see, choosing the right grit and grit sequence, while clearly important, can be a relatively tricky business. In this article we covered the basics of grit, what it is and the common labeling systems, the basics of what each grit will accomplish, and the Golden Rule of Sanding. Additionally, we discussed some tips for sanding various materials and applications, as well as some basic guidelines for selecting your specific sequence. As with anything as multivariant as sanding, trial and testing is the only real way to determine the best practices for your application.
If you’re a Uneeda customer and would like assistance in optimizing your process, contact us for support from our tech team.
If you’re just getting started with Uneeda, contact our sales team to receive product recommendations tailored to your needs.