Believe it or not, choosing the right sandpaper can be confusing because of all the different offerings available. Some of the differences include the types of backings, adhesives, and minerals used to create the sandpaper. Another difference is the anti-loading features: open coat vs. closed coat, and even super size coat. The construction of the abrasive (disc, belt, sheet, etc.) is pretty simple, as is the the way in which the sandpaper is attached to your sander (hook & loop or adhesive backing).
The most important thing to do before choosing an abrasive is to properly diagnose the job requirements and understand which paper will do the job correctly. Sanding jobs can include sanding primer, feather-edging, sanding metals, removing scratches, stripping old paint, cutting down filler, and finish sanding.
Backing - A flexible or semi-rigid backing (paper, cloth, film) to which the minerals are attached with adhesive. There is a designation for flexibility and durability ranging from A-weight (most flexible) to F-weight (least flexible).
Adhesive - An adhesive bond system is secures the abrasive mineral to the backing. All coated abrasive products are made using a two-stage bonding process: the make coat and the size coat.
- Make Coat - Initial layer of adhesive to anchor the mineral to the backing.
- Size Coat - Secondary layer of adhesive to lock the mineral in place.
Minerals - There are four basic types of minerals:
- Aluminum Oxide - Brown or blue, these are the industry standard for metal, wood, composites, and plastics.
- Silicon Carbide - Black in color, these are used for sanding primers, sealers and paints, non-ferrous metals, and final finishing.
- Ceramic Aluminum Oxide - A synthetic mineral that is very durable with extremely uniform crystalline structure. Examples: 3M Clean Sanding Discs (purple), and Norton Stick & Sand Discs (gold).
- Alumina Zirconia - Primarily for heavy stock removal of metal and wood.
Abrasives sometimes load-up with dust before the mineral is actually worn-out.
1. Open vs. Closed Coat: Open Coats have less minerals to create more space between abrasives to reduce loading. Closed Coats have 100% mineral application with hardly any voids. A closed coat abrasive should have a longer life—as long as it does not load up with dust, which is a major problem with composite and woodworking abrasion. Open coat abrasives have 1/3 less abrasives than closed coat abrasives.
2. Supersize Coat: There are some coatings applied to the abrasive that work as anti-loading agents (e.g., 3M Fre-Cut abrasives)
DIAGNOSING SANDPAPER PROBLEMS
The biggest complaint about sandpaper is that it wears out too quickly. But this could mean a number of things, and it's important to diagnose what is really 'wearing out'. Often times, it helps to pull your sandpaper out of the trash and really take a hard look at it (as funny as it sounds).
- 1. Is the abrasive mineral 'dulling' too quickly? You need a more aggressive mineral.
2. Is the backing falling apart? You need a more durable backing.
3. Are the minerals falling off of the backing? Your adhesive is failing.
4. Is the abrasive so clogged with junk that it no longer cuts? You need to look for an 'Open Coat' paper or maybe even one with an anti-loading agent (3M Fre-Cut or 3M Clean Sanding Discs).