Automotive sandpaper comes in a variety of different shapes, sizes and grits to be used for specific applications in the refinishing process. Actually Automotive grade sandpaper is completely different then what you might be use to in wood working. Wood workers or general purpose sand paper may actually be made of sand particles that are roughly graded into course medium and fine ratings. The abrasive particles on automotive sandpaper actually highly graded oxides to allow the auto body technician to depend on a contestant grade of abrasive.
Automotive Sandpaper can come in a variety of coarseness from about 40 grit which is the most abrasive up to 5,000 grit which is not much more abrasive then a regular sheet of paper in your computer printer. Each piece is exactly the same as the last that way if you are sanding down an imperfection in the clear coat you can grab a 3000 grit paper and know you won’t be cutting through the clear into the base coat.
So what are the different grades and types and when do you use them?
Starting with the most abrasive and working our way up to the fine grades.
40 Grit or anything below 100 grit is used to remove paint, rough sand and shape body filler and fiberglass, remove heavy rust or level out welds.
100 to 200 Grit is used to prepare metal with a rough surface so body filler will stick to it, detail sand the body filler and remove paint.
200 to 300 Grit is used to prepare the car for primer, finish sand body fillers and level out paint chips.
300 to 600 Grit can be used for finish sanding but is mainly used to prepare the Primer for the main Base Coat of paint.
600 to 1000 Grit is used to sand the base coat, remove large paint runs, dirt in the base/top coat or imperfections.
Above 1000 Grit is used to level the clear coat and remove orange peal this is the final process before buffing and may be skipped in some cases.
Now in any body shop you will find tools and products being used for reasons other then their main purpose but for learning sake the grades above are a good rule of thumb.
Rough sand with 100 or below to remove paint and shape the body filler. Once the shape is correct finish sand for primer using a 200 grit paper and once the primer has dried use a 400 grit sandpaper to prepair it for the top coats of paint.
If your car is straight and doesn’t need any body work you can start off with a 220 grit to prepair it for primer.
If you are repairing a dent and you want to match the paint around the damage you would start with what ever you need to repair the actual dent then you would sand the surrounding 6 inches to a foot all around the damage with a 400 to 600 grit sandpaper to to roughen up the clear coat and prepare the paint for a blend in of color coat. When preparing old clear coat for a touch-up you might even just use something called a scotch brite pad which is similar to the green scrubby things you have in the kitchen to clean pots.
As for Types of sandpaper there are 2 main types. Wet – Dry Sandpaper which is usually only used by hand and comes in grits above 100 and regular dry paper that will fall apart if you work with it wet. Dry paper is used on power and hand sanding blocks and is used at below 400 grit for paint applications.
The reason that you want to jump to Wet sanding when using grits higher then 400 is that the paper will clog with the material you are sanding. It is important to always keep the surface and the paper clean and wet so that a buildup of sanded paint doesn’t act as an abrasive.
Shapes and Sizes
Sandpaper packs come with 9″ x 12″ sheets which can be folded and torn to fit most standard sanding blocks and sanders. To do this you fold the long side into 3 even parts then tare it against a straight edge like a counter top. Cutting sandpaper with a knife or scissors really doesn’t work very long as it will dull the blades but if you have to cut on the paper side.
Sandpaper in packs or lose sheets can be used for hand sanding or for power sanders.
Sanding disks come in packs of 5 or come on a roll of 100 if you are purchasing yours at a body supply shop they can sell you just the number you need off of a roll which is much cheaper then buying packs.
The most common size is 6″ to fit DA Sanders and the largest variety will be found in that size however some companies do make larger orbital sanders that come up to 10 or 12 inch which are really nice for leveling out large flat areas.
For other sanders you can also buy 2 to 4 inch wide rolls that work well with inline sanders that are long and rectangular. This reduces all the time needed to cut pieces from sheets and some sheets won’t be long enough for larger sanders. Inline sanders are used for leveling wide flat or rounded but even areas on doors, fenders, hoods.
Specialty Products
There are always new and better products comming to market and some of them don’t last while others catch on. As mentioned above there is a product called a scotch brite scrub pad surface abrader. This product is not technically sandpaper but it will replace sandpaper in locations like door, trunk and hood jams. They are easy to use and will cut down the clear enough that you can band in with a detail gun. The biggest reason they are good for these areas is that they are used wet and they will provide a good surface and remove residual dirt.
You will also see a number of small disk sanders that use a screw in application or a velcro type attachment. they are good but more expensive then the regular adheasive disks. The smaller ones are often used for metal conditioning along with a die grinder.
Overview
For standard painting projects you want to work with your metals and fillers with a dry sandpaper up to about 200 grit then for your primer to adhere you can use a mixture of wet dry on painted areas and disk paper to cover large areas. Once the primer is on it is best to use wet sanding to level the primer however shops will normally use an orbital disk sander. After the paint is on the car and you have a run or imperfection you need to let it dry at least a full 24 hrs or use a IR paint drying lamp and then wet sand only with grits above 400.
The YouRepair Store has a variety of Autobody tools and Sandpaper for your next project. Remeber that most orders over $25 can ship free with super saver shipping.