Introduction
Sanding is, at its core, a material-removal process. Every grit stage in an abrasive progression removes a
measurable layer of material, leaves behind a scratch pattern characteristic of that abrasive’s particle size,
and requires the next finer grit in the sequence to remove the scratch pattern left by the one before it. This
is true whether the substrate is bare metal, painted coating, or composite. Understanding this
mechanically — not just as a matter of technique, but as a matter of physical material science — is
essential to understanding why resurfacing, no matter how far the grit progression is carried and no matter
how fine the final polish applied afterward, remains fundamentally a different category of process from
non-abrasive refinement.
Abrasive Grading Standards
Abrasive grit is classified by average particle size under standardized grading systems, most commonly
the CAMI (Coated Abrasives Manufacturers’ Institute) system used predominantly in North America, and
the FEPA (Federation of European Producers of Abrasives) “P-grade” system used predominantly in
Europe and increasingly worldwide, referenced in FEPA Standard 43-2. These systems are not perfectly
interchangeable at every grit level, and a “220 grit” designation under one system does not always
correspond exactly to “220 grit” under the other — a detail worth confirming when specifying or comparing
abrasive products across manufacturers or regions. Coated and non-woven abrasive grain sizing in North
American industrial contexts is also referenced under ANSI B74.18.
Grit Progression Mechanics
Coarser grits (lower numbers, e.g., 220) remove material relatively quickly because their larger, sparser
abrasive particles cut deeper individual scratches into the surface with each pass. This speed comes at
the cost of leaving a correspondingly deep and visually obvious scratch pattern. Each subsequent, finer
grit in a progression is used specifically to remove the scratch pattern left by the previous, coarser stage
— not to add anything to the surface, but to progressively reduce the depth and visibility of scratching until
it falls below the threshold of visual detection under normal lighting conditions.
A typical progression on metal moves from coarse grits (220–400) through intermediate grits (600–1000)
to fine grits (1500–3000 and beyond), followed by a mechanical or chemical polishing stage intended to
remove the final, finest micro-scratch pattern that even the last sanding grit leaves behind. It is worth
stating plainly: every single step in this chain removes material. There is no stage of a sanding
progression, however fine, that adds material back to the surface or leaves the surface’s original outer
layer intact. Even the final polishing stage, which may not use a bonded or coated abrasive in the
conventional sense, is typically still removing a very fine layer of material through mechanical or
chemo-mechanical action — this is the core mechanical distinction between resurfacing, which by
definition includes an initial sanding or grinding stage, and refinement processes that are capable of
improving surface reflectivity without ever removing the original surface layer to begin with.
Consequences of Material Removal
Removing material through sanding has several concrete, measurable consequences distinct from — and
often overlooked in favor of — its cosmetic outcome:
l Dimensional change: the finished part is measurably thinner after sanding than before it began, in
every location the abrasive contacted. On components with engineered tolerances — aerospace
skins, pressure vessel shells, structural fittings, or any part manufactured to a specified thickness
range — this is not merely a cosmetic consideration. It is a dimensional change relative to the part’s
original manufacturing specification, and on safety-critical or load-bearing components, uncontrolled
or undocumented material removal can have implications for the part’s certified service life or load
rating that go well beyond appearance.
l Removal of the original surface layer and any associated protective film: mill-finished and rolled
metal surfaces carry characteristics — including, on many stainless alloys, a passive chromium-oxide
layer, and on aluminum, a native oxide film — that are specific to the original manufacturing process
and are permanently removed once the surface is sanded through, requiring re-formation (natural or
chemically assisted) of an equivalent protective layer afterward.
l Introduction of a new scratch pattern requiring full correction: each grit stage must be fully
worked out by the subsequent, finer stage, or residual scratch patterns from an earlier stage remain
visible, or become effectively “trapped” beneath a later polishing step — a common and
well-recognized defect in improperly executed grit progressions, sometimes visible only under certain
lighting angles after the work appears complete.
l Heat generation and thermal effects: aggressive sanding, particularly with power tools and at high
grit-removal rates, generates localized frictional heat that can, in some materials and conditions, affect
the microstructure of the surface layer or accelerate degradation of adjacent coatings; controlling heat
input is a standard consideration in abrasive machining practice generally.
Sanding and Distinctness of Image
As detailed in EXQUISITEMAD’s companion articles on gloss versus distinctness of image (DOI), sanding
is the primary — and, for macro-scale surface topography, effectively the only — tool available for
increasing DOI on a metal or coated surface, because it is capable of physically flattening the larger-scale
surface topography (waviness, mill lines, orange peel) that governs DOI, in a way that no non-abrasive
polishing process can replicate. This is precisely why sanding produces a fundamentally different surface
outcome from non-abrasive polishing even when both can be brought to a comparable specular gloss
reading, as demonstrated in EXQUISITEMAD’s stainless steel panel study: one process changes the
surface’s underlying geometry and thickness in a permanent, irreversible way; the other does not touch
either.
Knowing When Sanding Is, and Is Not, the Appropriate Tool
Because sanding always trades material and dimensional integrity for improved macro-scale image clarity,
the decision to sand should be made deliberately, with the trade-off explicitly acknowledged, rather than
defaulted to automatically whenever a higher level of surface perfection is desired. A surface with an
already-acceptable macro-scale profile, but with reduced fine-scale reflectivity due to environmental wear
or light contamination, is generally a poor candidate for sanding, since non-abrasive refinement can
typically restore its reflectivity without incurring any of the costs described above. A surface with genuine,
significant macro-scale defects — deep pitting, heavy scoring, or warping — may require sanding to
achieve a meaningfully improved result, but that decision carries a real, permanent cost to the part’s
remaining material and, in some cases, its certified specification, and should be weighed accordingly,
particularly on industrial, aerospace, or structural components where material loss is not merely cosmetic.
Limitations and Disclaimer
This article describes general abrasive mechanics and is educational in nature. Appropriate grit selection,
progression, and whether sanding is appropriate at all depend on the specific material, its condition, its
engineered tolerances, and its intended service. No guarantee of results is made, and no warranty,
express or implied, is provided or endorsed. This article does not constitute professional or engineering
advice. Consult a qualified professional and verify material specifications and tolerances before
performing any sanding or resurfacing procedure.
References
1 FEPA Standard 43-2:2006, Grains of Fused Aluminium Oxide, Silicon Carbide and Other Abrasive
Materials for Coated Abrasives — Macrogrits P12 to P220, Federation of European Producers of
Abrasives.
2 ANSI B74.18, Abrasive Grain Size for Coated and Non-Woven Abrasives, American National
Standards Institute (aligned with CAMI grading).
3 ASTM D523, Standard Test Method for Specular Gloss, ASTM International.
4 ASTM D5767, Standard Test Method for Instrumental Measurement of Distinctness-of-Image Gloss of
Coated Surfaces, ASTM International.
Note: Citation details above should be independently verified against the issuing body’s current designation listing
prior to reliance in a formal or professional context.