Wet-Sanding (Colour Sanding)-

Some methods described may be beyond the capabilities of the average detailer or enthusiast; in this case we strongly encourage the reader to consult a professional detailer or body shop concerning the matters discussed herein. The writer assumes no responsibility, expressed or implied, due to misuse or misinterpretation of the information or methods used, or for any vehicle damage or injury that may occur due to the suggestions and information offered.
Color Sanding / Polishing: Years ago it was called colour sanding because automotive paints were single-stage (primer and a colour coat) and when the finish faded from ultra violet radiation (UVR) exposure, sanding off the oxidized paint thereby exposing the underlying paint would bring back the paint surface colour. Then, as now, most sanding was done wet so the terms colour sanding, wet sanding and as all these process involves a sanding block, block sanding all became interchangeable whether you were sanding for colour or for a level surface.
The secret to a Concours d’elegance winning paint finish is color-sanding (before wet-sanding paint show vehicle owners often have an extra 2.5 mils to 3.0 mils of clear applied).
This kind of work should only be undertaken by a very experienced enthusiast or a professional detailer; experienced in colour-sanding (wet-sanding) more so than detailing, as this is more a paint renovation technique that takes years to perfect.
The paint sections / panels that are difficult and involve risk are usually those that are close to the edges of a panel (as this is where the paint is usually thinnest) or highly contoured areas. Flat areas like the trunk, hood or roof involve minimal risk.
Wet-sanding levels the paint (and removes orange peel, harsh acidic or alkaline marks, etc) it removes approximately 0.025- 0.003 Mils (a Mil is 1/1000th of an inch) of paint from the vehicle using 2000 -grit finishing paper, Bear in mind that a clear coat has a thickness of 1.5 – 2.0 Mils, removing more that 0.3 – 0.5 Mils of clear coat may cause premature paint film failure. As a point of reference a sheet of Saran wrap measures 0.95 Mils
A digital paint thickness gauge will allow you to measure the film thickness of the paint system which includes the e-coat, primer, base coat (colour) and clear coat, total thickness is usually 4.0 – 6.0 Mils. Check the paint system both before and after wet sanding and this will tell you how much paint has been removed.
For these reasons all car manufacturers now specify that the paint thickness be measured in 0.1mil, or 1/1000th of an inch before and after any wet sanding or buffing. The following are the maximum allowable clear coat reductions the major USA car manufacturers will allow; Chrysler- 0.5 mils, Ford – 0.3 mils, GM – 0.5 mils
Paint thickness gauge - PosiTest DFT Combo; Measures paint on any metal, including steel and aluminium, a popular choice given many cars have both steel and aluminium. It automatically recognizes the material and takes a measurement- Paint Gages, Paint Meters, Mil Gages for measuring Paint Thickness on Automotive Cars, Trucks and Vehicles.
* Sanding Prep- wash paint surface thoroughly to remove any surface contamination, fill a spray bottle with paint cleaner/ degreaser (DuPont PrepSol II™ – http://www.xurex.com/products.html) to remove old wax and soluble contaminates, spray onto a 100% cotton micro fibre towel (DF Towels – DF Concours products) and clean the surface. If you don’t wipe the surface before you sand you will not remove the contaminants, you will force (sand) them into the paint surface

* MethodologyPut a drop or two of Woolite® liquid in a clean bucket and fill it with clean water. You will need 6-inch x 4-inch sheets of Nikkens™ 2000, 2500 and 3000 – grit finishing paper (1000 and 1500 grit is also available, but they are usually used on single stage paint systems, you can start with any grade of finishing paper according to the condition of your paint film surface, but as always use the least abrasive first) next cut or tear three or four pieces of the finishing papers in half and place them in the bucket also.
Leave them in the bucket of water for about 20 -30 minutes to soften their edges. Keep the surface wet at all times and sand in short strokes, applying light to moderate pressure with your hand or block held flat.. After going over the finish with 2000- proceed to 2500- and then switch to 3000-grit finishing paper and sand once more.
While you’re waiting for the paper to soften, wash your vehicle too completely to remove any grit or dust, and then gently dry it with clean terry cloth towels. Next, take a tack rag and wipe over the paint surfaces one final time to make sure the paint is absolutely clean. A small particle of grit can make deep scratches in even the hardest finish. If you’re working outdoors, wet down the surrounding area to keep dust under control, and keep a garden hose running with a trickle of water on the surface at all times.
Wrap a piece of the softened Nikkens™ 2000 – grit finishing paper (or whatever grade your paint surface requires) around a Unigrit Sanding Block (see also alternative products) if you’re going to be working broad, flat surfaces. Otherwise, you can just fold the sandpaper in thirds. Keep wetting the surface of the vehicle in the area where you’re sanding and keep it wet while you work. Place your hand flat on the sanding block / finishing paper at all times and start sanding in short, crisscross strokes. Avoid applying excessive pressure, keep the sanding block flat or you’ll gouge the paint surface and never sand with just your fingertips, as you’ll create grooves if you do.
Only move the finishing paper and block about 4 – inches back and forth, if you do make a scratch, it will be a short one and easier to buff out. Using shorter, quicker strokes requires about the same amount of time as the longer sweeps most people are used to. Besides, you shouldn’t worry about time and have a lot of patient when color-sanding. A clean rubber squeegee (3M™ Wetordry™ Rubber Squeegee, P/N 05517, 2 -inches x 4 -inches) is the best tool for checking your progress. Swipe the squeegee across the sanded surface periodically to clear it of water. Only when you have the surface completely free of any suggestion of orange peel or imperfections are you ready for the next step. Proceed to the next level of grit and repeat the process until all the coarser scratches are eliminated and finish with 3000 – grit finishing paper in order to remove all of the very fine scratches, only when the paint has a consistently level, smooth, satin finish should you continue to the polishing stage.

• Alternative – 3M™ Trizact™ Hookit™ II Clearcoat Sanding Disc – P1500 (PN 02069)- large area wet-sanding, runs damp, long lasting, excellent for texture matching and sanding contours, removes nibs and other defects, leaves a very refined finish

* Going through the grits - What does this mean?” It’s a very similar idea to ‘Least aggressive first’ it refers to the process of using different grit finishing papers of progressively finer grit or foam / wool pads with progressively denser pads to get a smooth finish (i.e. using finishing papers 500 grit increments will yield flatter results – 1000 > 1500 > 2000 > 2500 > 3000) By going through the grits each progressive piece of grit finishing paper or pad removes the scratches from that previously used. Always start with the least abrasive combination , i.e. a machine polish and the least aggressive foam / wool pad. If that combination doesn’t remove the imperfections, using the same machine polish and change to a more aggressive pad, if the surface imperfections remain, then step-up the abrasive polish and use the least next level (abrasive)only when the paint has a consistently level, smooth, satin finish should you continue to the polishing stage.

• Final polishing (Menzerna Final Polish II) with a grey / black foam pad, speed 1500 RPM

Bi-directional sanding- if you are sanding in a \\\ pattern with the 2000, then sand in a /// pattern with the 2500, once all your lines are opposite then you know you have completely covered the area. (See also Polishing, Spit Shining, Colour, Depth and Clarity, Legal Stuff, Wet Sanding / Polishing)
Note- It is possible to wet-sand orange peel, but if you try to eliminate it completely you risk severely compromising the paint thickness. (See also Legal Stuff)
* Note:

1. Aluminium Oxide exhibits a high degree of friability. This means that when heat and pressure are applied it fragments. This is highly desirable because when it creates new sharp edges, this self-renewing property allows aluminium oxide to last longer than most other finish papers.
2. Finishing paper quality – cheaper paper will induce deeper sanding scratches, which will be more difficult to remove