Guide 2: Choosing Sandpaper: What Grit to Use?

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Guide 2: Choosing Sandpaper: What Grit to Use?
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- Bigger grit number = smaller particles size = finer grade = smoother  surface = less visible scratches = higher gloss = more light reflected = more mirror-like finish
 
- Use Micron-Graded abrasive, e.g. 5 micron film or cloth instead of non-graded P5000 paper for:
-High precision finish for consistent result
-Uniform scratch pattern & higher gloss
-Lesser sanding steps & faster sanding
 
How do I know if an abrasive product is graded? 99% of polishing products on eBay are non-graded. Only a few sellers sell those products. Graded abrasive will be stated as “micron-graded” or given an exact grit size in micron or micrometer. Most paper-backed abrasives are not graded, in other words, they are FEPA-graded (P-Grit), which have higher size tolerance than Micron-grading system. Most film-backed abrasives are graded.
Do not use non-graded polishing paste or liquid abrasive on surface that had been polished with graded sandpaper / polishing film as it can cause damage & scratch
Use all consecutive grits e.g. P1000-P1200-P1500-P2000 instead of skipping grits for:
-Best surface finish
-Shortest sanding time
-Longest lasting abrasive
You can skip 2-3 grits for lower grit e.g. P40-P80-P120
It is acceptable to skip 1-2 grits, e.g. P400-P600-P1000
It is recommended not to skip grits from P1500 onwards (at most skip 1 grit)
Polishing Films require less grits than sandpaper due to the fact that most of them are Micron-graded and thus give more uniform result, eliminating the needs of close consecutive grits
Suggested main grits: 40, 80, 120, 240, 400, 800, 1200, 1500, 2000, 2500, 3000, 9000, 11000, 14000, 60000 or 40, 80, 120, 240, 400, 600, 1000, 1500, 2000, 2500, 5000, 9000, 11000, 14000, 60000
So if you want to get Grit 120 to 3000, you can get Grit 120, 240, 400, 600, 1000, 1500, 2000, 2500 & 3000
- Your coarsest grit must be able to remove the original surface scratch
 
- Start with finer grit to remove scratch on very thin material (e.g. car paint & jewellery) to prevent excessive removal of material
 
- For help on choosing grit or sandpaper product, use:
-Search engines e.g. Google, google e-book
-Other general sites e.g. eHow & All Expert
-Automotive manufacturer & detailing forums, e.g. Meguair’s forum & Detailing World
-Hobby forums, e.g. wood finishing, car scale model, precious metal clay
-Abrasive manufacturers education or information section e.g. Mirka & 3M website
-Ask seller who specialised in the material being polished e.g. jewellery seller
 
Same grit of sandpaper does not always give the same surface smoothness as it depends on:
-Particle sizes grading – Micron-graded film will always give higher gloss than sandpaper (non-graded) at same grit
-Mineral type – Diamond film will give better result than Aluminium Oxide due to its extreme hardness & shape
-Abrasive type – Cushioned sanding cloth give better smoothness due to special cushion effect of its backing, e.g. P500 cushioned Sanding Cloth will give result equal to P800 Sandpaper
-Manufacturers - Specialist companies such as Mirka, 3M & Micro-Surface produce their abrasive to better quality control & higher consistency in particles sizes grading, use higher quality material and follow the specifications required for Grits more closely, so will give better result
If you are still unsure, get P120 to P2000 to cover most of your sanding needs
During sanding, it is assumed that half of the abrasive particles penetrate the surface being polished. So, for 40 micron particles, 20 micron deep scratches can be produced.
Even though grit such as P2500 can produce mirror-like finish, under strong or direct light, you still can see fine scratches. That’s why you will need higher grit to remove those scratch pattern so they will not be visible.

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Sandpapers and other abrasives are all available in different smoothness to suit different needs.

The most commonly used standardised unit is Grit Number, which is the number of mineral particles in 1 square inch of surface, in simplest term. (Don’t want to bore you with the details on exactly how they measure this) So the bigger the Grit, the more particles are there in once inch square surface. In other words, the higher the Grit number, the smaller the particles, and thus the finer (higher) the Grit. And the finer the Grit, the smoother the polished surface will be, generally.

The sizes (diameters) of the sanding particles (called grit/grain) is called Grit Size. It is measured in micrometer (μm), or incorrectly but commonly known as micron. 1 Micrometer is equal to 0.001 mm (milimeter). Human eyes can only see objects as small as 40 micrometer but that doesn’t mean you can get perfect scratches surface by sanding up to P400 as scratches a small as10 micrometer, for instances are visible for naked eyes due to light scattering or shadows effect. Remember that the higher grit, the more light the surface can reflect, thus the higher the gloss - brighter.

However, different countries, manufacturers or even products use different systems so there are a few different Grit systems. Most of the sandpapers in market are using the International Standard, which is based on FEPA (Federation of European Producers of Abrasives) P- Grit system for coated abrasive. For this system, all the Grit will start with letter "P", e.g. P240. The Grit number is written on the back of most sandpaper. Even when the letter “P” is not written, you can almost always assume it use FEPA P-system. Officially, the whole range of P- Grit sandpaper is P12 to P2500, but a few manufacturers are now producing sandpapers beyond the range e.g. P8 to P5000. As FEPA does not have standardised Grit designation for beyond P3000, different manufacturers use their own systems when assigning the Grit, resulting in some confusion to what their actual particles sizes are. My grit designation for my Grit Chart for beyond P3000 is constructed using input from 3M and some mathematical equations to calculate the grit.

Not many companies have Grit beyond P2000 & P2500 and that is why P3000 is more expensive. The only manufacturer I know that have paper-backed sandpaper with Grit beyond P3000 is “Matador” from Germany which has P5000. I am not sure about its Grit size though as there is no such information available. They are a few companies from China & Korea that produced P4000 and P5000 though. I haven’t got the chance to test their quality yet. There are, however, a few companies that produce film-backed sandpapers beyond P5000, up to 0.01 Micron! As these very fine sandpapers are harder to find and manufacturing processes are more expensive, they are much more expensive. On the other hand, very coarse sandpapers (coarser than P40) are also very hard to find and are thus more expensive too. These P24-P40 sandpaper are usually used for rapid removal of floor material.
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