How To Wetsand Paint Series by Mike Phillips
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This is an article on
damp-sanding, which is part of a series of articles on the topic of wet-sanding. Damp-sanding is a variation of wet-sanding except that you work by machine and use dramatically less water.
Wet-sanding, Color-sanding & Damp-sanding
First lets start by clarifying some terms as some of you reading this will no doubt already be familiar with some variation of sanding automotive paints and just to note, wet-sand and color-sand, or wet-sanding and color-sanding, are interchangeable terms.
Color Sanding
The term color-sanding came from the time cars were primarily painted with single stage paint and as such when you sanded them dry or with water you would see the color of the paint coming off the car, thus the term color-sanding. If you were wet-sanding a pigmented or single stage paint, your water slurry would be the color of the paint.
Wet Sanding
Wet-sanding is just a term to describe sanding automotive paints with water. If you're wet-sanding a single stage paint you'll see the color of the paint in your water slurry as you sand. If you're sanding a clear coat finish your water slurry will be a whitish color as the sanded particles of clear paint are not clear but whitish or opaque and thus the water slurry will have a whitish appearances to it.
Damp Sanding
Damp-sanding is simply a variation of wet-sanding except that you use less water with sanding discs designed for use with a process that uses less water. The water slurry from damp-sanding will either be whitish if you're sanding a clear coat finish, or the color of the paint if you're sanding a single stage finish.
Water Slurry
The water on the surface of your work area that suspends particles of paint removed off the surface from the abrading action of your sanding paper or sanding disc.
Now onto the article...
Damp-Sanding Tools, Tips and Techniques by Mike Phillips
Damp-sanding is a machine sanding technique. Damp-sanding is like wet-sanding only you use a lot less water, basically you use just enough water to maintain lubrication between the paint surface and the face of your sanding or finishing disc. It's important that you maintain the right amount of water because if you use too much water your paper will
hydroplane and if you use too little water the face of your sanding disc will load-up and become clogged with paint particles and you'll wear out the abrasives prematurely.
The difference between Wet-sanding, Color-sanding and Damp-sanding
Traditionally
air-powered sanders called D.A. Sanders are used because they offer speed and power plus they provide a more uniform sanding mark pattern which is important because this helps to make it dramatically easier to remove your sanding mark.
Another way to damp-sand is using an
electric D.A. Polisher. It is important to note that at this time there are
no electric polishers or sanders that are approved or recommended for use with water to either wet-sand or damp-sand. The reason for this is there are no electric sanders or polishers that can pass the "Wet Test", this is where a strong spray of water is blasted at the tool from different directions.
This article does not recommend using an electric polisher to damp-sand, in fact I would recommend using an air-powered D.A. Sander to be on the safe side. That said, I have used an electric polisher many times to damp-sand automotive paints and so far I have never had any issues with shock hazards. I also recommend making peace with your Maker before anytime you'll be using power tools of any type.
Of course keep in mind damp-sanding is using only a small amount of water from a spray bottle and a lot of common sense. Also anytime you're using any electric tool around electricity you should always have the tool plugged into a
GFI outlet.
Why sand paint?
The reason most people would either undertake sanding down the paint on their car or paying someone to sand down the paint on their car is to create a flawless, show car finish. A show car finish is a finish that is completely clear and/or defect free. A show car finish offers excellent depth, clarity, gloss, shine and maximum D.O.I.
In order to create these characteristics, the paint must be completely flat and free from any orange peel as well as any other above or below surface paint defects. When we use the term flat to describe paint we don't mean as in a matte or low gloss finish, we mean the surface is completely flat like a piece of window glass. When the finish is completely flat, it will reflect crisp, clear images like a mirror with no distortion, this is referred to as D.O.I. or Distinction of Images. To get an automotive finish completely flat or level, the paint is normally sanded flat and then buffed to a high gloss.
Refinishing Industry vs OEM
Generally speaking, the majority of all sanding done to automotive paints is done in what is referred to as the Refinishing Industry, that is in layman terms, your local body shop or custom paint shop. OEM stands for Original Equipment Manufacture, which in layman terms means car manufacturing plants, the place where new cars, trucks and s.u.v.s are built and assembled and at some point in their assembly... painted.
Fresh Paint vs Factory Paint
Throughout this article we will refer to either
fresh paint or
factory paint.
- Fresh paint is just that, it's when someone has taken their pride and joy to a body shop, or custom painter and paid to have a custom paint job sprayed onto their car.
- Factory paint is the paint that came on the car from the factory.
The primary difference between the two types of paint are,
- Thickness or thinness of the topcoat
- Hardness or softness of the topcoat
Here's an excerpt from another article I've written on the topic of wet-sanding...
Wet-sanding - Fresh Paint vs Factory Paint
Fresh paint sprayed at a local body shop
Fresh paint will tend to be softer
Modern basecoat/clearcoat paints are catalyzed, that is they are chemically hardened, a simple analogy would be the way you mix a 2-part epoxy glue together and the resulting product hardens through a chemical reaction. If you've ever worked with fiberglass and polyester resin where a small amount of catalyst, (hardener), is added to the resin and the chemical reaction cause the resin to cure and harden, that's another simple analogy as to how catalyzed basecoat/clearcoat paints are hardened.
Contrast this to older style paints which were called solvent-evaporation paints in which the paints dried without the use of a catalyzing agent and instead simply dried and hardened over a longer period of time as the solvents mixed into the paint evaporated.
Even though most modern paints are chemically cured, there is still a window of time where these paint are not 100% hardened and during this window-of-time the paint is what is sometimes refereed to as still wet, not like in wet gooey paint but as in still soft enough to easily sand and buff.
There are also what are called fast drying, medium drying and slow drying reducers which are solvents for mixing in with the paint to give it a thinner viscosity before spraying. A painter can match their choice of reducer to speed up or slow down the drying time.
All these factors can affect paint hardness or softness in the first few days that follow after the car leaves the paint booth.
Usually, if the painters knows the car is to be wet-sanded after painting he can adjust how he mixes the paint to give the person usually called "The Painter's Helper" time to sand and buff the paint before it becomes too hard. Time of year which includes temperature and humidity can also play huge factors in drying or curing time and can be factored in and adjusted for.
Now here's what's key about this, sanding paint is always easy whether the paint is fresh or baked-on at the factory, in simple terms, (very simple terms for the purpose of explanation), sanding paint is putting scratches into the paint.
Again... sanding paint is easy, it's the part where you try to remove your sanding marks that can be difficult. The harder the paint the more difficult it will be to buff out your sanding marks. The softer the paint the easier it will be to remove your sanding marks.
Fresh paint will tend to be thicker
At the body shop level, again if the painter knows the car is to be sanded and buffed, they'll usually add an extra coat or more of paint, this gives the painter's helper a little more wiggle room to sand and buff and not worry about sanding or buffing through the clear coat and exposing the basecoat.
I've also met painters that will just spray a thicker or heavier coat and only spray 2 coats of paint but through factors they can control they can spray it on thicker and therefore not have to spray a third or fourth coat.
Fresh paint will be sprayed on custom projects with more planning and higher expectations.
If you're having a custom car project painted, for example you spent months and more than likely years rebuilding a classic Mustang and now it's time to get it painted, in most cases you'll be meeting with the painter ahead of time planning out the paint job and it's at this time you discuss with them your expectations. If you want the car sanded flat and then buffed to a high gloss for a true show car finish, (if this is you goal or expectation), then the painter will spray an extra coat or two of paint to provide plenty of film-build for the painter's helper to safely sand the paint flat. They will also build in the cost of the extra materials, (clear paint), time and labor to your bill.
Summery
Fresh paint sprayed at your local body shop, specifically the clear layer, will tend to be thicker than the clear layer that comes from the factory and it will tend to be softer and easier to sand and buff shortly after the car comes out of the paint booth. Because it's thicker there is a little more safety margin or wiggle-room for the person to sand and buff the paint and not break-through the clear layer and expose the basecoat. Because the paint is fresh it's going to be softer than factory baked-on paint, at least for a window of time and this will make buffing out the sanding marks faster and easier.
Factory baked-on paint
Factory paint will be harder
The original paint sprayed onto your car as it traveled down the assembly line at the manufacturer's plant is in most cases baked-on at high temperatures before any wiring or the interior is installed and for this reason higher temperatures can be used since there's nothing to melt of catch on fire in or on the car yet. By the time the car pops-out the end of the assembly line the paint is fully cured and hardened. For this reason it will still be easy to sand, (that's putting scratches into the paint), but it will be more difficult to remove your sanding marks out of the paint.
Factory paint will be thinner
At the factory, the paining process is very automated and the amount of clear paint applied to the vehicle is done so in a tightly controlled manner and to very stringent specifications. You don't have the ability to ask for an extra coat of clear, or for a thicker coat of paint to be sprayed and from a materials cost point of view, it's probably safe to say that the amount of paint used to coat each car trends towards being the minimum amount, not a generous amount. Simply put, factory clear coat paints tend to be very thin compared to what you can get at your local body shop.
Summary
Factory paint will tend to be hard and thin, it will be easy to sand but more difficult to remove your sanding marks out 100%
Factory paint will be thin and if you're not really careful you'll break-through the clear layer and expose the basecoat or color coat either during the sanding process or the ensuing buffing process as both procedures remove a little paint.
Damp-sanding Fresh Paint
In a body shop situation in most cases when the painter knows the car will be sanded for a show car finish then they will spray extra clear, (or extra paint in the case of a single stage finish), onto the car to provide more material or film-build for sanding and buffing. For these situations, you can start with the more aggressive 1500 Sanding Discs to level orange peel and remove surface texture.
Most sanding for a custom paint job is done while the paint is still soft or fresh, that is within days of the car being moved out of the paint booth, thus the term fresh paint.
Damp-sanding Factory Paint
Because the factory finish on new cars, truck and s.u.v.s are
thin, sanding and compounding should only be used to correct serious below surface defects as an
option and if you have confidence the paint has not been sanded or compounded previously and of course, confidence in your skills and abilities.
The theory behind using Meguiar's Unigrit Sanding and Finishing discs to sand thin, factory paint is that while it adds a step in most detailing processes, it saves time overall because it enables you to remove the defects faster by requiring less passes with your rotary buffer and aggressive compounds. This reduces heat, enabling you to buff cooler and can also help to reduce deep swirls.
Without sanding, you don't know how deep the defects are so you don't know how aggressive you need to get in order to remove them, the only way to find out is by trial and error. With Unigrit Sanding and Finishing Discs,
you have control over the depth of the sanding marks thus you have more control over the compounding step.
Keep in mind that all the UV protection for the basecoat is suspended in the clear coat or clear layer of paint sprayed over the basecoat or colored layer paint. You need approximately 1.3 mils of clear to provide enough UV protection to protect the color coat from failing. New cars come with approximately 2.0 mils of clear paint so you have a little wiggle room for sanding and buffing but always follow the practice of,
"Use the least aggressive product to get the job done"
Damp-sanding factory paint usually mans sanding on very hard paint because the paint was baked-on at high temperatures at the manufacturing plant and is no longer fresh and could be anywhere from months to years to decades old. This means the paint is fully cured, fully dried and fully hardened.
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Originally Posted by jamores23
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