How to Block Sand a Car

by Tom Keaton

One of the steps involved in body work preparation before a car is painted is a system called "block sanding".This is an all important step which allows you to obtain very flat smooth surface before the paint is applied.All high end paint jobs go through the block sanding process which insures that excellent flat finish!

Step 1

Body repairs complete on this fender

Block sanding takes place after any body damage has been repaired and fillers have been applied, such as bondo or glazing compounds, and the car is in primer. There still may be occasion to perform some minor body work and filling if the block sanding uncovers flaws.

Step 2

Pick out the proper sanding block for the job. If you are working a flat area, use a flat and long sanding block. For a rounded contour, use a rounded block that will best fit the contour.

Step 3

Black primer is guide coat

Spray a dark colored primer over the gray. This darker coat of primer is called a "guide coat" and when you sand the dark primer off, any low spots will be determined by the gray primer below it that is not effected by the flat surface sanding.

Step 4

Flat block sander for flat surface.

Wrap the sanding block with the 220 or 320 grit paper and begin to sand the area in even steady motions over the primed surface. Keep your block "flat" to the surface, changing directions as you go along and wiping up the sanding dust with a clean dry cloth. For a large flat surface you can use an x direction sanding pattern and then move from side to side. The object is to discover if there are any low or high spots which will show up through sanding. Much of this technique is getting the feeling or touch to sand deep enough to show any flaws or low spots but not so hard that you cut too far down and create more body work for yourself. A low spot will show the light primer below and a high spot will generally show bare metal as you sand over the area.

Step 5

Fill low spots with filling compounds such as final putty or glazing compound. High spots will have to be either sanded down, grounded or pounded back to get flat to get the high spot to become a low spot, which would then be easy to fill. After any filling compounds are dry, sand the repair area with a coarser grit paper wrapped on the proper block, possibly as coarse as 120 grit and follow up with 220 or 320 afterwards. Then spray new primer in the appropriate color and repeat step 4 until the surface is very flat and smooth.

Once the surface is flat and your block sanding has not unveiled any low or high spots you can work your way to a finer grit paper of 320 and then finally 600 grit which will leave a nice finish for final primers and sealers.


  • -When "feeling" the results of your block sanding, which is suggested, put a paper towel or thin cloth between your fingers and the body or wear a cloth glove which will allow you to feel smaller imperfections you may want to repair. Using a cloth will also keep oil off the finish.
  • -Be patient and repeat sanding and repairing as often as needed. Any flaws left to remain will be amplified once paint is applied to the car.


  • -Don't use your fingers to work the sandpaper or you will leave uneven impressions, always use a block using steady evenly pressured strokes.
  • -Never allow any oil, grease or silicone on the primer . The surface you are working should be dry and clean at all times.

Items you will need

  • Various shape and size sanding blocks
  • Primer in spray cans (2) different colors, one dark and one lighter
  • Clean cloth towel or rag
  • Paper towels
  • 220-320 and 600 grit sandpaper sheets
  • Filler compounds (Optional, if low spots are discovered and need repair)

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