Hong Life CalculatorsUnderstanding the sandpaper grit sequence planner — concept and principle
This planner lists the sandpaper grits to use between a starting grit and a finishing grit, so that each paper removes the scratches left by the one before it. It is used for preparing wood for finish, smoothing filler and primer, and refinishing furniture, where skipping too far ahead leaves scratches that show up under stain or varnish.
How it is calculated
The calculator holds a standard list of grits: 40, 60, 80, 100, 120, 150, 180, 220, 240, 320, 400, 600, 800, 1000, 1200, 1500, 2000 and 3000.
- It starts at the starting grit.
- Each next step is the finest standard grit that is at most 1.5 × the current one (and not past the finishing grit).
- It repeats until the finishing grit is reached and shows the sequence from coarse to fine.
The guiding rule is not to jump more than about 50% in grit number between steps, using as few papers as that allows.
The principle
A grit number describes particle size: the higher the number, the smaller the abrasive grains. Under the FEPA P-scale, for example, P80 grains average about 0.20 mm, P120 about 0.125 mm and P240 about 0.059 mm. Each grit cuts scratches roughly as deep as its grains. A finer paper removes material slowly, so if the previous scratches are much deeper than its own grains, it takes a long time to remove them, and some survive. Moving up in moderate steps means each paper only has to erase slightly larger scratches, which is the fastest route to a uniform surface overall.
Worked example
- Inputs: start at 80, finish at 240.
- From 80, the limit is 80 × 1.5 = 120, so the next grit is 120.
- From 120 the limit is 180, so 180; from 180 the limit is 270, and the finest grit up to the finish is 240.
- Result: 80 → 120 → 180 → 240.
How to read the result
The list is the shortest ladder that stays within the 50% guideline. You can add the in-between grits (100, 150, 220) for a gentler climb, but it is rarely necessary. Start coarse only when there is real damage, glue or old finish to remove; on fresh, planed wood 120 or 150 is often enough. For most stained or oiled wood, 180–240 is a common final grit; going much finer can reduce stain absorption.
Common mistakes
- Jumping from 80 straight to 240, which leaves 80-grit scratches visible under finish.
- Mixing scales: FEPA P-grades and CAMI (US) grades are similar in the middle range but differ at the fine end.
- Not wiping off dust between grits, so loose coarse grains keep scratching.
Frequently asked questions
What grit should I use before painting?
Bare wood is usually sanded to around 120–180; between paint coats a light pass at 220–320 helps adhesion.
Why do "P" and plain numbers differ?
P marks the European FEPA standard; unmarked numbers often follow the US CAMI standard, which uses different particle ranges for fine grits.
Sources: FEPA Standard 43-1, Grains of fused aluminium oxide and silicon carbide for coated abrasives (P-grades) · ISO 6344, Coated abrasives — Grain size analysis