How-To Damp-sand
Here are the basic techniques for damp-sanding
Preparation
Wash OR wipe the vehicle to remove loose surface dirt
Wash car to remove all loose surface dirt off the paint and out of any cracks, crevices, body seams and trim. It is vitally important that the vehicle is surgically clean before sanding because if any abrasive particulate enters the sanding process and is trapped between your sanding disc and the paint finish it will instill what are referred to as
pigtails.
Pigtails - Photo courtesy of Mister B
Pigtails are deeper circular scratches that look somewhat like a curled pig's tail thus the name. Typically what happens is an abrasive particle gets trapped between the paint and the face of your sanding disc and because the backing plate that holds the sandpaper is moving in an oscillating fashion, it grinds the abrasive particle into the paint in a curly pattern that looks somewhat like the tail on a pig, thus the name.
Pigtails
Work Area Size? - 2" x 2" or smaller
In the real world, you would slice up the car panel by panel and then slice up the panel to section usually no larger than about
20” squarish or so. This is not a hard and fast rule because not all panels are square and flat, in fact since the 1980’s most car bodies incorporate aerodynamics designs and are very rounded and curvy.
Why you don't want to tackle too large an area at one time...
Regardless of the shape of the panel, the reason you don't want to tackle too large of an area at one time is because you have better control over downward pressure when the sander is near your body. The further you reach out away from your body the more likely you'll change the downward pressure and possibly tilt the polisher potentially causing more pressure to and edge on the face of your sanding disc and this can cause gouging or deeper scratches in the paint.
The width of your shoulders is a pretty good measurement for as large of a dimension as you would ever want to tackle on a surface. My shoulders measure approximately 20" wide so I would only want to tackle a section 20" squarish or smaller, not larger.
Water Volume?
Damp-sanding is using only a
few mists of water. Meguiar’s new Unigrit Sanding and Finishing Discs MUST be used wet but not over-wet. All you need is a mist or two of clear water to the face of the pad and a few mists of water to the surface to be sanded. Damp-sanding should not be a wet-mess on the car, or on the ground surrounding the car.
- Too much water will hyper-lubricate the surface and reduce cutting action.
- Too little water will increase cutting, clog disc and wear disc out prematurely.
Unigrit Sanding and Finishing Discs are
always used wet. Use water only - no sanding lube (soap).
Use a clean water source - no soap or sanding lube
It's safer to error on the side of caution and use an extra mist or two of water until you get a feel for the right balance for your environmental conditions. Temperature, humidity, airflow, affect all machine sanding process including the Unigrit Sanding System so there can be no hard and fast recommendations. Practice makes perfect.
VITALLY IMPORTANT
It's vitally important that you stay focused on the amount of water you're using when damp-sanding because if you use too much water and your disc hydroplanes, you can mis-read this to mean the disc is spent of used-up and no longer cutting.
If the disc is in fact still good and you throw it away because you used too much water then you waste a good disc and these are not inexpensive.
Remember, it's not wet sanding it’s damp-sanding
I took the below picture after completely sanding down a 1969 El Camino. Note the water level in the clear spray bottle.
Only 16 ounces of water used to sand an entire 1969 El Camino
This is a 32 ounce spray bottle and I never refilled it during the sanding process, this means I used approximately 16 ounces of water to sand down a full size, domestic vehicle.
Downward Pressure?
Light Pressure. When machine sanding you only need to use the weight of the machine and whatever more pressure it takes to keep the pad flat to the surface with no hopping or skipping. It’s vitally important to maintain disc rotation so you don't want to use too low of a speed setting.
Tool Speed Setting?
Fast. With an air powered D.A. Sander you're going to be on 3/4 to full throttle With a Meguiar's electric polisher you’re going to use the 3.0 to 6.0 speed setting depending upon whether you're sanding a flat surface or a curved surface. It's safer to turn the speed down anytime you're sanding small curved areas or hard to reach areas.
You can see the Throttle next to my thumb on the underside of this 3M Air-Powered DA Sander
Air Pressure or PSI
Before using an air powered DA Sander check to see what the tool manufacture recommends for Maximum PSI or PSIG.
PSI = Pounds per square inch
PSIG - Pounds per square inch at the gauge
I think the main difference between PSI and PSIG is that PSI is not specific to the point of connection for the tool and could mean compressed air pressure at the tank and not the actual gauge and thus the air line feeding your tool. That means there can be differences in PSI by the time it gets to your tool depending upon the shop you work in and how many lines are being fed by the air tank/compressor. The important point is to make sure you don't starve your air tool for compressed air and you don't over feed the tool with too much air pressure.
The 3M DA Sander shown in this article states to not exceed 90 PSI
The air regulator for use in our shop is usually set to around 110 PSI so when I want to use the air compressor I have to decrease the pressure coming out of the line by adjusting the regulator down to 90 PSI
Sanding Pattern?
Cross Hatch Pattern --> UMR = Uniform Material Removal.
The goal when sanding is to remove the defects while removing as little paint or topcoat material as possible. Of course the reason for this is to leave as much paint on the car to last over the service life of the car. So your specific goal is to only sand enough to remove the defects and then
STOP. The only way you will know how much sanding is required to remove the defects is by
testing each paint system you work on and getting a feel for how many passes it takes to remove the defects you're going after.
Because topcoat hardness is a variable you cannot control, you must do a Test Spot on each new car you work on to figure out how easily or difficult the paint is sanding to remove enough material to reach your goal.
Cross Hatch Pattern
Sanding back in forth in a straight line with a DA Sander creates the potential to remove material unevenly if you're not very careful. Adding a second direction will help insure the surface remains flat with no low/high troughs or valleys running a dedicated direction in the panel. A cross hatch pattern helps to ensure
uniform material removal over an entire panel.
Of course the shape of some panels will not lend themselves to sanding in a cross hatch pattern and you will be limited to only sanding in one direction. For panels such as these be sure to overlap your passes by 50%
Pre-determined pattern = Easy to duplicate and less thinking required
The benefit to using a cross hatch pattern is that it's easy to remember and thus easy to
duplicate over and over again for production sanding work. Feel free to come up with your own system that suits your goals best, these are just my recommendations as to what works best for me. If you have employees, you always want to make procedures they will do over and over again easy to learn and repeat for fewer mistakes.
Arm Speed?
Use a
rapid arm speed when machine sanding.
Unigrit discs are designed to cut fast and cut for a long time with light pressure and a fast arm speed. Moving the sander/polisher too slowly will cause the abrasives to cut too deeply and will wear the disc out prematurely. There’s a learning curve to every process and that includes machine sanding with these new Unigrit Sanding Discs and you’re better to error on the side of caution by using too fast of an arm speed versus too slow.
When first starting out, stop often and check your results.
Number of passes?
When sanding with the Meguiar's Unigrit Sanding Discs, after the initial break-in of the disc, sand for
4 - 6 Section Passes and then inspect.
The definition of a pass
There are two definitions of the word pass as it relates to machine polishing with any type of machine. We can apply this to machine sanding as well.
Single Pass
A single pass is just that. It's when you move the tool from one side of the section you're sanding to the other side of the section you're sanding. That's a single pass.
Section Pass
A section pass is when you move the tool back and forth, or front to back with enough single overlapping passes to cover the entire section one time. That's a section pass.
The definition of a pass
There are two definitions of the word pass as it relates to machine polishing with any type of machine. We can apply this to machine sanding as well.
Single Pass
A single pass is just that. It's when you move the tool from one side of the section you're sanding to the other side of the section you're sanding. That's a single pass.
Section Pass
A section pass is when you move the tool back and forth, or front to back with enough single overlapping passes to cover the entire section one time. That's a section pass.
Big Picture
Sand a section of a panel until you have removed the orange peel and any other surface texture or surface imperfections. Then move to a new section on the panel and contour until the entire panel is sanded flat.
Next move onto a new panel and repeat.
Sand until the entire car is sanded flat.
Next you want to give the car a complete and thorough wipe down using a water dampened microfiber towel to make sure any loose abrasive particles are removed. If any sanding abrasive particles get trapped between your buffing pad and the paint when you go to buff out the sanding marks this can lead to deep swirls. So be sure to work clean.
Next you'll want to,
- Compound the paint using a wool pad, cutting compound and rotary buffer.
- Polish the paint using a fine cut polish a foam pad and your choice of a rotary buffer or some type of dual action polisher.
I'm going to guess that the goal for your 1955 Chevy is a show car finish, if that's the case then I'll guess you don't want any holograms in the paint left by a rotary buffer.
The easiest way to ensure there are
zero holograms it to use a dual action polisher for your LAST machine polishing step.
I'd also recommend using the best compounds and polishes your budget will allow. The most important aspect of the finish results for ANY type of machine polishing project is the
abrasive technology.
Hope the above helps, if you have any further questions please post them and myself and the rest of the AGO forum members will do our best to answer them as quickly as possible.
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