Abrasives for Automotive Paint Prep

Posted: March 10, 2014 By: kensium

Many times I have heard folks say “ah, sandpaper is sandpaper, what’s the difference?” Well, that may be somewhat true if you are only concerned with making a nice pine birdhouse, paper towel holder or a magazine rack for your wife’s birthday that is just fine but that term just doesn’t apply here.

There is a broad difference in abrasive papers specifically used in automotive painting and not knowing the difference can result in a lot of aggravation and great disappointment when doing an automotive paint project. What we are going to make some sense of in this month’s article is the confusing array of abrasive papers available today. Essentially, most of it is hard, sharp abrasive material, usually aluminum oxide or silicon carbide, bonded to a specialized paper backing with waterproof resins and adhesives however it comes in many shapes, sizes, methods of attachment and grit.

Grit of course, is a standardized method of grading the size of the minute particles stuck on the paper. The higher the grit number, the finer the abrasive particle. A 180 grit will be much courser than 400 grit and 1000 grit will be much, much finer than a 400. In determining a grit size for abrasive paper, the manufacturer will allow up to that particular size particle but nothing larger to be applied to the backing. This ensures that the particular size scratches made by a particle are all of one size and smaller. Scratches? Yes, that is essentially what the abrasive paper is doing. Using literally 1000’s of hard, sharp particles with the help of friction to wear away a surface either in paint removal with some of the courser grits like P80 or P120 or finer grits used on bare metal and or primer in prepping for paint like a P320 or P400. It is also always helpful to envision that each successively finer grade of abrasive paper is simply “wearing away” the courser scratches made by the courser grit used before it. This is why final “wet” or “color sanding” is done over the final clear coat or other finish painted surface with a P1000 or even P1500 grit paper leaving a glass-smooth surface. Under a microscope you would still see sanding scratches but to the eye it looks perfectly smooth.

While on the subject of grit, one other very important item worth noting is that there is actually more than one grading system for abrasive paper grits. In the US, the two most commonly seen grit rating systems are; the US CAMI (Coated Abrasive Manufacturers Institute) usually expressed in a plain number such as 400 and the other, the European FEPA (Federation of European Producers of Abrasives) “P” grade systems preceded with a “P” as in P400. It can be rather confusing as both systems are nearly identical in particle size in the courser grits such as 80 or 180 however at around 400 as the grits get finer; the variation between the rating systems gets wider. To make the issue even more confusing, many suppliers today only offer the “P” graded paper however; and have dropped the preceding “P” in their product descriptions. Very Important! Keep this mind when buying abrasive paper. If you aren’t sure, just ask about what you are getting before you buy. Oh, and quality…you usually get what you pay for in buying abrasive paper. Don’t cut corners here.

The attached table is useful in determining the proper grit size for a particular application and helps to better explain the variation between systems. 

Abrasive Paper Grit Size Table

ISO/FEPA Grit designation

US CAMI Grit designation

Average particle diameter (µm)

MACROGRITS
Extra Coarse (rarely used in automotive painting)

P12

1815

P16

1324

P20

1000

P24

764

24

708

P30

642

30

632

36

530

P36

538

Coarse (rapid removal of old finish and filler, also shaping of fillers)

P40

40

425

50

348

P50

336

Medium (sanding bare metal in preparation for finishing, for gentle removal of old finish)

60

265

P60

269

P80

201

80

190

Fine (sanding bare metal in preparation for finishing)

P100

162

100

140

P120

125

120

115

Very Fine (sanding of bare metal before primer)

P150

100

150

92

P180

180

82

P220

220

68

MICROGRITS
Very Fine (sanding sealer/filler primer before final primer)

P240

58.5

240

53.0

P280

52.2

P320

46.2

P360

40.5

Extra Fine (sanding of primer before finish)

320

36.0

P400

35.0

P500

30.2

360

28.0

P600

25.8

Super Fine (initial “wet-sanding” or “color-sanding” of final finishes)

400

23.0

P800

21.8

500

20.0

P1000

18.3

600

16.0

P1200

15.3

Ultra fine (final “wet- sanding” or “color-sanding” and polishing for mirror like finish)

P1500

800

12.6

P2000

1000

10.3

P2500

8.4

 

Size and shape: Although there are many odd sizes, the three basic common abrasive paper shapes are rectangular, long strips and round discs.

  •  Standard, rectangular 9” x 11” wet or dry sheets that can be cut or torn to the size desired or purchased pre-cut to fit ¼ or 1/3 sheet sized sanding blocks or simply folded in quarters for hand sanding.
  • The long strips are 2-3/4” wide to fit rigid or flexible long boards and can be packaged in pre-cut 16-1/2” lengths for manual & powered long board sanders or torn from a roll as needed to fit longer foam type flexible board sanders. Note: Long boards are typically metal or wood based with handles and used for sanding large expanses of car body. Some are flexible, high-density foam to allow following of gentle curves. These can be from 6 inches to 3 feet long. Powered long boards are generally 15-3/4” x 2-3/4” and air powered. There are electric powered units available and they are fine for dry, paint removal and filler sanding but since electric and water don’t work well together, they are not a great idea for wet-sanding.
  • Lastly are the round discs which are most commonly found in 6” diameter to fit “DA” or Dual-Action and other powered rotary sanding machines although 3”, 4” & 5” can also be found. Note: Dual-Action sanders operate with a dual rotary and orbital pattern or can be mechanically locked in a rotary only motion. As with the powered long boards, these are usually air powered and although electric versions can be found and work well for dry paint removal work, I don’t recommend wet-sanding with them.

Means of attachment: The three most common are:

  • Dry backed which requires clips or other mechanical means of attaching the abrasive paper to the sanding tool. This is usually found in the rectangular sheet material and long board strips and rolls. Generally used in all grits of paper from the coarsest to the finest.
  • Another is “PSA” paper or Pressure Sensitive Adhesive backed with a peel-off backing sheet covering the adhesive. The backing is stripped off, the abrasive paper is stuck to the tool face and when worn, is simply pulled off and discarded. This is commonly a choice for the long strips or rolls and the round disc papers and requires a tool with a smooth, flat facing surface capable of accepting the self-stick adhesive. Most often used in the finer grits of abrasive paper although it is available in most any grit.
  • One other method of attachment is hook and loop backed although this is usually only an option for round, disk type abrasive paper and can be somewhat more expensive and then usually in the courser grades. Of course it will only work on a compatible hook and loop base pad and is sometimes found on higher RPM rotary machines and smaller diameter rotary tools where PSA backed paper lacks sufficient gripping ability.

There are many, many novel configurations of sanding blocks and sanding boards; some powered by hand while others are electric and pneumatic. There are rotary machines and DA (Dual-Action orbital and rotary) and each type has a specific purpose. Some highly experienced guys’ will sand with a sheet of paper folded in their bare hands and do a perfect job while others will say it is an absolute no because it is impossible to achieve a level sanded surface. Since there are so many variables and surfaces to consider, there is actually no one tool or method that can be chosen as “the best way”. Some experienced people will prefer one over the other and each will do a perfect job. If you are planning a paint job in the future, pick up a few good books and DVD’s on surface prep, do your homework to see what is available today in sanding equipment. If you can, try out different methods and tools to see what feels best for you.

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