Body Shop Basics: Grits & Grinders
Handling our own bodywork and/or preparation for paint can be easily accomplished at home with a minimum investment in tools and materials. Pictured here are all the tools and materials you will need to handle a basic body repair such as a small dent or high spot. Having compressed air is a luxury for some of us working out of our garages. While most pro body men and painters prefer pneumatic power, electric or cordless grinders and sanders are more practical and affordable for home use. One of the prohibiting factors of doing body repair or paint prep at home is the lack of compressed air. Fortunately, companies such as SEM offer good quality aerosol undercoats for bodywork and paint prep, eliminating the need for spray guns and an air compressor.
Whether it's a very basic body mod such as a bolt-on spoiler or body kit, or a more advanced procedure requiring repair to a dented panel, all bodywork and paint prep involves the use of a variety of abrasives. Part of the key to saving money is learning how not to waste it by knowing what the different types of sandpapers are for and what grits are used for preparation.
One of the first steps to successful and cost-saving DIY bodywork is to familiarize yourself with sandpaper grits as well as types of sandpaper and grinding discs. There is a purpose for each grit and type of sandpaper but you'll be glad to know that you won't have to buy them all. Below is a rundown of the more common paint and body sanding grits and what they are most often used for.
The Cheesegrater: Sometimes referred to as a rasp, this inexpensive tool is not really an abrasive but it is the most effective tool for shaping body filler. Knowing how to use the grater is an acquired skill similar to block sanding, and using it the right way will save time, effort, and abrasive materials after you begin to sand. The grater can be used for rough-shaping and for removing the glaze on the surface of catalyzed body filler that clogs sandpaper.
24-, 36-, 40-grit: Although a coarser 16-grit is available, 24-, 36- and 40-grits are usually coarse enough to handle most material removal requirements. A variety of sizes are available but small repairs can be performed with a 3-inch and a twist-on or pressure-sensitive adhesive (PSA) 3-inch pad on a 90-degree die grinder or cordless drill. Larger areas can be worked with a larger disc on a high-speed sander or a random orbital sander with 6-inch pad.
80-, 100-grit: These are intermediate grits that are often used for light material removal and final-shaping of body filler prior to applying any type of primer. When shaping body filler, the repair spot must be perfectly straight before proceeding to finer grits. The transition from body repair to paint prep occurs at the point where grits in this range are used.
120-, 150-, 180-grit: Feather-edging is the process of leveling the existing layers of a finish where grinding has occurred. Always begin with the coarser grit first, and gradually progress to finer grits as the quality of leveling improves. Trying to feather a coarse paint edge with 180 first will only result in smoothing out the edge, not leveling it. 120-grit is the best grit for leveling layers of existing paint and undercoats.
220-, 240-, 280-, 320-grit: Again, working from coarse to finer, these grits are used for block-sanding primer coats to eliminate the subtle waves and imperfections from a panel. The condition of the body and also on the paint color (darker colors, pearls, and metallics tend to show more imperfections than light, solid colors) will determine how much block sanding will be required.
400-, 600-grit: These grits are considered final-prep grits for sanding undercoats, OE sealer, or an OE paintjob prior to paint. The type of basecoat you plan on using will determine what grit to use. Remember that waterborne basecoats have a thinner dry film buildup and require finer sanding grits. Solvent-based urethane basecoats and lacquers can be prepped with 400- to 500-grit, and acrylic enamel topcoats can be prepped with 220- to 320-grit.
Sandpaper grits in the ultra-fine range are used primarily for cutting and buffing to break down any unwanted texture in the finish, referred to as orange peel, or remove any tiny dust particles that may have found their way onto the wet surface. This is followed by compounding and polishing with a buffer. Originally, grit numbers were a rating system that indicated how many grains of the abrasive would fit along a one-inch line. If only 36 grains could fit on the line of reference, it was labeled 36-grit. If you look at and feel a sheet of 36-grit paper, the grains are large. By comparison, you cannot see or feel the individual grains on a sheet of 2,000-grit paper.