Abrasives

 

Sanding . . . More than meets the eye or hand

Look around and most everything you see has been sanded during its manufacture.  Coated abrasives, commonly referred to as sandpaper (and sometimes sanding sponges), are widely used throughout the world in industrial applications.

What are the components of sandpaper?
There are three primary components of sandpaper:

  • The backing - which provides a foundation for the grain.
  • The adhesive bond - which holds the grains onto the backing.
  • The abrasive grain - which is the "cutting tool" that chips away unwanted material - much like a metal saw.
What is backing?
The backing can be paper, cloth, fiber, or sponge . . . depending on the use.  Paper backed products include sanding sheets, discs and rolls.  Paper weight varies by as much as six different levels.  Cloth is more resistant to tearing and can have greater flexibility.  Because sanding belts are subjected to more abusive applications, they are usually backed with cloth.  Some applications are so demanding that a fiber backing is required, as paper or cloth would tear.

What holds the "sand" to the "paper"?
An adhesive binds the "sand" to the "paper".  Two general types of adhesives are used:  animal hide glue and synthetic resins.  Adhesive is applied in two applications:  the maker coat which anchors the abrasive grain to the backing.  The second is the size coat which is applied after the maker coat has dried.  The size coat is applied over the grains, flowing them and providing final adhesion.  Resin bonds are more resistant to heat, more durable, and waterproof.  But, because they will not break down as quickly as glue bond, they yield a rougher finish.  Generally, paper backed products use a glue bond.  Especially for fine finishing applications, a glue bond product works well. 

How does the abrasive work?
The abrasive grains work together as thousands of miniature cutting tools on a piece of sandpaper.  When a grain gets dull, a section of the grain will fracture or splinter to expose a sharp new cutting edge.  So, the friction, pressure and heat of a sanding operation will make the grains "self-sharpening".
What is grit?
Grit is a name given to the size of the abrasive grains.  Grains that are too large can damage the work-piece and grains that are too small can cause needless time and effort being spent.  Basically, grit size is determined by passing the abrasive grains through a series of screens that contain openings of carefully controlled sizes.  The more openings per linear inch in the screen, the smaller the abrasive particle size.  Grit size varies from 12 (Extra Coarse) to 600 (Extra Fine) with about 17 grit sizes in between.  The most used grits are coarse - 80, medium - 100 & 120 and very fine - 220 which represent over 3/4 of the product used.

Are there different types of abrasives?
Yes, they include:  Aluminum Oxide - a man-made mineral grain, grayish tan in color, is typically used in high production painting job sites.  It is considered an all-purpose product because it can be used on metal, wood, plastic and painted surfaces.  And, Garnet - a natural abrasive, reddish-orange in color, is popular in woodworking and often used for critical fine finishing and sanding between coats of paint.

What other abrasives may be found?
 Silicon Carbide - commonly found in non-loading and floor sanding products is made by fusing silica sand and carbon, dark green or black in color, with a sharp splintery crystal-shaped grain.  And, Emery - another natural abrasive that is blocky in shape but weak, black in color, used in metal sanding products.

What is the significance of the letter A, C, or D that follows the description of the grit?
Usually 9" x 11" sandpaper sheets are available in three paper backings.  "A" weight is good for handheld use and close work.  "C" weight is good for handheld use but will stand up to some machine usage.  "D" weight is very good on a sanding machine and a little stiff, though usable, in the hand.

What are the steps in the typical sanding process?
Sanding is usually done in steps, using three to four grits in the process.  The first step is complete "rough-sanding" of the entire surface, typically using 60 or 80 grit "D" with a power or machine sander.  Always sand with the grain, not across it.  The second step is again complete sanding of the surface, but with a "finer" grit such as 100.  Step three (and sometimes step four) are done with 150 (and 220 grit).  After the last sanding step, vacuum the dust away and wipe the surface with a tack cloth before applying any coating.
Why does sandpaper curl when it is stored and how can the curling be reduced or prevented?
If you think of sandpaper as a "laminated product" with paper on one side and resin and small rocks (abrasive grain) on the other side it becomes easier to understand the tendency to curl.  The grain and resin do not absorb moisture.  However, the paper and animal glue are able to absorb and release moisture.  In humid conditions, the paper takes on moisture and expands . . . causing curling with the abrasive inside.  In dry conditions, moisture is lost from the paper causing it to shrink and curl with the abrasive on the outside.  Curling can be reduced by storing the sandpaper in its original container.  If possible, 70 degrees and 50% relative humidity is the ideal atmospheric condition for storage and anything close to this will minimize curling. ALLPRO offers top quality sandpaper in contractor and handy packs which are re-closeable and great for storing the sandpaper.