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Author Topic: Painting  (Read 1084 times)
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Sham Courtney
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« on: June 02, 2011, 10:38:51 PM »

Found this on another forum over here, might come in handy for people.

Some paint terms

Flash time: How long a coat of paint must be allowed to dry before another coat of paint can be applied.

Open time: How long a container of paint can be exposed to air before its integrity is compromised.

Pot life: How long a mixed batch of paint can set before its integrity is compromised.

Reducer: Thinners used with paint. There are different temperature ranges of reducer, so you have to pick the right one for the temperature range of the environment in which the paint will be applied.

Activator: A catalyst added to paint which will cause the paint to set, or cure.

Overspray: Paint that does not go where you intend it to.

Primers

Etching primer

This is a primer specifically formulated for bare metal adhesion and is used on areas where unprimed metal are present. If you have small areas, smaller than a fist, you can probably get away without using this primer. This primer can be top-coated with one of the following primers prior to the color coat. Although it is possible to top-coat with color, you should follow the manufacturers recommendations for the primer and your color paint.

High build primer

High build primers are primarily used where extensive bodywork has been done, involving body fillers. The high build primers contain a lot of solids and will fill in a lot of voids, slight imperfections and sanding scratches in body filler and old paint. Most high build primers involve a 2 part system, paint and activator. The nice thing about this is that it’s easy to mix and since there are no reducers involved, the paint will lay down nicely over the top of other paints and bodywork without any adhesion issues. The drawback to high build primers is that you need a large orifice gun to shoot them, around 2mm, and unless they are laid down properly, will leave a rather rough finish which will require a lot of wet-sanding prior to receiving a top coat. The top coat for high build primer should be a primer sealer or epoxy primer, both of which are discussed later. Some high build primers can be mixed in a 3 part system with reducer on the final coat, and act as a primer sealer. You can also shoot color coats over the top of high build primers but you should follow the manufacturers recommendations for the primer and the color paint.

Primer Sealer

Primer sealer will act as a barrier between old and new, and keep the reducer in your paint from biting into your old paint, bodywork, etc. which can compromise adhesion. The difficult part about using a primer sealer is that is has to be painted over with your color coat during a specific window of time, otherwise the paint won’t adhere properly and will need to be sanded, and sanding the primer sealer compromises the sealing properties of the primer. This window of time will vary between manufacturers but is usually 6-8 hours. If you exceed that window, you’ll want to wet-sand using 600 grit, rinse and dry, and then shoot another coat of primer sealer on. Then shoot your color coat on after the paint has flashed, but before the time window has closed.

Epoxy Primers

This kind of primer is very durable and usually has a lot of solids in it. Epoxy primers can be formulated for bare metal adhesion and as a high build primer surfacer. Typically coupled with an activator, most can be reduced as well. They are generally more expensive than other primers but can be well worth the cost.

Primer Surfacers

Most primers are considered primer surfacers, in that they are applied to create a uniform, homogenous surface for a successive coat of another product.

Flex additives, fish eye reducer, and adhesion to aluminum, yellow metals and plastics

There are a lot of products available for this. Some are paint prep products, paint additives and others are a primer that is painted on. Flex agents are becoming a thing of the past, as are fish eye reducers, which were used when paints were lower solid content and involved lots of reducer. Most of the major market contenders in automotive finishes have excellent products to deal with these surfaces.

Paint Guns and Compressors

HVLP, siphon, gravity feed, pressure pot and airless

HVLP stands for High Volume Low Pressure. HVLP guns cost more than others and involve an improved means of atomizing the paint which allows the operating pressure to be lower, while still applying a decent amount of paint, which reduces overspray significantly. There are HVLP guns that come in various configurations, although typically it is gravity feed.

Gravity feed guns have the paint cup on top and use gravity as well as the venturi suction of the air coming into the gun, to mix the paint and air together. They operate on relatively low pressure and have less overspray. These are the easiest guns to clean, use, and are the least expensive.

Siphon feed guns have the paint cup on the bottom and rely entirely on the venturi suction to pull the paint out and mix it with the air. Siphon feed guns require higher pressure to make this happen, and in turn, they typically have more overspray.

Pressure pots basically remove the paint cup from the gun and utilize a hose from the gun to a large can where the paint is placed. This type of gun is expensive, requires a lot of cleanup and can limit mobility.

Airless paint sprayers are typically not used for automotive paints. They use only the paint to propel itself out of the gun, use higher pressure than most and have a lot of overspray.

As for Air Compressors, you need a large capacity, quick recovering one to paint a car. Each time the compressor runs, that's more hot air you've got to take the moisture out of, and the pressure at your gun will fluctuate significantly when the capacity isn't sufficient. So the higher the pressure capacity, and the higher the volume capacity, the more CFM of air it can deliver at the operating pressure of your gun without having to compress more air, and the easier it is going to be to paint since you won't constantly have to watch your pressure gauge and keep adjusting it.

Sand paper

Most sand paper is paper backed and is meant to be used dry for paint removal and for shaping of plastic body fillers. It ranges in grits from 36 to 320. Grits 400-3000 are typically meant to be used wet for preparation of paint for successive coats of paint. The water cleans the paper and the surface being sanded, which reduces sanding scratches which will transfer through the paint. I only use grits below 120 for shaping body filler and for removing rust. Grits above 120 and below 400, I use for feathering the edges of body filler and the edges of paint that meats bare metal. 400 and 600 I use for wet sanding primers and paint prior to application of another paint. Anything above 600 I use for sanding imperfections out of clear coats prior to buffing with compounds and a machine.

Block sanding and guide coats

Block sanding is very important to proper paint prep. All areas that have been repaired with body fillers must be block sanded. This is done with a hard rubber block that has nails embedded in the ends to hold the paper in. A person can also use a long wooden sander for larger areas or a pneumatic version of that. After a repaired area has had it’s first few coats of primer, it should be block sanded again to make sure the repaired area will not be visible once painted. An easy way to identify low and high spots is to apply what is referred to as a guide coat. This is usually accomplished by spraying a very light coat of a different color primer. It’s applied almost like a mist so that the paint has a peppered look. Then the whole car will be block sanded and the mist removed. Areas that are high the first coat of primer may be removed entirely, and in low areas both the primer and the guide coat will be visible. Those areas would be reworked and repainted with primer followed by a guide coat and block sanded again. When wet sanding, a soft foam pad should be wrapped with the paper for sanding inside and outside corners and curves. For large flat areas, a rubber pad is used. Wet sanding should be done with a large bucket of warm water and a sponge, constantly rinsing the area with clean water and rinsing the paper. You know you’ve gone far enough when the paint is dull. Use a large sponge and a bucket of water to keep the area very wet, and re-soak the area frequently, rinsing paint off and cleaning your paper out. Use moderate pressure and let the paper do the work, pressing hard just wears you and the paper out that much faster and also puts heavier scratches in the paint. When you’ve got the whole car wet-sanded you’ll want to rinse it with clean water several times and towel it dry, you don’t want any water to dry on the car, it’ll leave deposits. You’ll want to shoot the car with a few coats of an epoxy primer or a primer sealer prior to your color coat for a high quality repaint but if you’re on a tighter budget or just doing a quick repaint you can do it without. If the car is going to be repainted professionally and will be driven during the work, or repainted at a later date, a primer sealer would be a wise decision to keep the paint sealed. The sealer would need to be sanded and re-shot prior to color coating.

Painting and paint prep

Preparing for paint is the entire process we’re discussing here, but more specifically, what to do between coats, between different paints and before painting your color and clear coats.

Masking

I use masking tape and newspaper to mask off, as well as trash bags for wheels if they’re on the vehicle. You need to mask of everything you don’t want to get paint on because overspray gets everywhere, places you couldn’t imagine it going. There’s really no trick to this but it’s cut and dry, you either do a good job or you do a bad job. The biggest thing is making sure that what you’re putting your tape on is clean and dry. If you’re working on a spot repair on a panel, mask everything but that panel. When you’re done you’ll probably need to buff the overspray off that panel, but that beats wet sanding a paint edge where the primer met your masking tape.

Overspray

To keep overspray from being a big issue, I wet the floor down and I rewet it if it dries out. You can make a paint booth inexpensively by covering your garage walls with plastic sheeting and I’m not talking about that thin painters plastic, you want the heavy 3 mil or heavier stuff because you’re going to be blowing compressed air around and have air circulation, so the last thing you want is thin plastic flapping around, knocking overspray off itself into your wet paint, or the plastic itself getting into the paint. You can also make a paint booth by simply framing up a structure using lumber and stapling the plastic to that.

Ventilation, respirators, paint suits, driers/filters and tack cloths.

Ventilation

If your garage has a window or outside door you can put a box fan in the opening to exhaust air as well as a cheap furnace filter to allow clean air in. You’ll need to make sure that only clean air can enter through the filter, so you might have to make baffles out of cardboard or plywood. If your overhead door is your only means of exhausting air and bringing in fresh, you can stop the door about 2’ off the floor. Tuck towels and old linens in the gap at the top, put your furnace filter on one end and your box fan at the other and fill in the middle with a frame covered with plywood or cardboard.

Respirators

You need to wear a respirator that fits properly and has filter(s) that are designed to filter out the chemicals in the paint. The mask should fit snug on your face and if you block the exhaust hole you should be able to pressurize the mask by exhaling and not have it leak. You should also be able to breathe in and not smell anything. To test this, you put the mask on and then go find something like perfume/cologne, aerosol air freshener, or something with a potent smell and make sure you can’t smell it.

Paint Suits

You can buy inexpensive paint suits at most paint suppliers. This will keep you clean and keep the paint clean of you.

Driers/filters

When an air is compressed it gets hot, which allows it to hold more moisture. To keep the moisture out of your paint you need to use a desiccant drier. The best place for a drier is as far away from the compressor as possible. This allows the air to cool, and the water will remove itself from the air, although it’s still trapped in the air hose. You can use a bucket of cold water and coil up a length of hose in it, and this will help to cool the air, and place your drier after that. You can also use a disposable desiccant drier right at the gun, as well as a filter.

Tack Cloth

You can buy Cheese cloth, which is just a really thin, soft cloth that won’t lint. You can also buy Tack cloth which is basically Cheese cloth with a resin applied to make it tacky. You have to be careful when using a tack cloth because if you press down on it it’ll leave the resin on the paint. So you want to ball it up and use a very light touch. You can use these between coats of paint if you allow enough flash time and you should always use them before applying your next stage of paint.

Painting

Most paints, be it primers or base and clear coats, say to use 2-3 medium wet coats. A medium wet coat is wet enough that it looks smooth and glossy but doesn’t run. You’re not going to cover on your first coat, so don’t try, and you might not even do so on your second, just make sure each coat is glossy with no runs and it’ll come out fine. I typically put 1/3 to ½ of my spray pattern into my previous pass, so that my coats overlap and it rewets the previous coat enough that the overspray doesn’t make it hazy. If you get a run in the paint, leave it alone, you can wet sand it out before you do your next coat, clear coat, or if you’re using a single stage, on you’re doing your clear coat, you can wet sand it out and buff the paint with some compounds. If you get a bug, lint or some other thing in the paint you can leave it alone or attempt to pull it out, it’s really got to be your call. To sand between coats, you need to wait until the paint is well past its flash time and feels dry to the touch. Then you’ll need to use a tack rag or cloth to clean the sanded area. You want to paint from the top down because as well all know, shit falls from the top down, and your overspray will do the same. You want to start your successive coats in the same general area that you started the previous one. Hold the gun as close to perpendicular to the surface as possible and move quickly on corners, paint builds quickly there. Don’t stop your spray flow until you’re past anything being painted. If you have lots of overspray, you can blow the car off with air before applying the second coat and if you’ve allowed enough flash time, you can lightly wipe the paint off with a tack rag.
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Driving my '88 Super Carry around Wellington, New Zealand....
RASCAL NI
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« Reply #1 on: June 05, 2011, 10:50:12 PM »

Good lad...the section on sanding has given me food for thought.
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spacehopper
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« Reply #2 on: June 08, 2011, 06:48:02 PM »

Missed out the important terms though - 'Brush', 'Roller', 'Pound Shop' and 'NATO Green'.
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RASCAL NI
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« Reply #3 on: June 09, 2011, 09:33:46 PM »

Your dead right...I did a test run with the roller on my tailgate...

2 Coats zinc primer, 3 coats satin topcoat, each coat smoothed off with 1000 grit & final coat smoothed with 2500 grit just to get rid of the dust speckles.

I really took my time to see what could be achieved & its absolutely impossible to determine wether its been rollered or sprayed...hope I achieve the same result overall.
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