Autobody 101

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I plan to make a series of posts to this topic over time that will help with decision making for body and paint work. I won't go into great detail, but sufficient enough so that it's relatively straightforward for the average person. I'll start this installment with a glossary of terms, not in alphabetical order.

BODY FILLER - Commonly called 'mud' in the trade, and bondo by most enthusiasts. This is what you use to restore body lines, contours, repair small dents, etc. It is sold under many names, but basically, it's the same product consisting of a thick viscous material mixed with a hardener or catalyst.

FINISHING FILLER - Sometimes referred to as 'glazing putty' but be careful not to confuse it with non-catalyzed filler which was actually called glazing putty. A finishing filler should use a hardener just like regular filler. If it doesn't come with a catalyst, don't use it. "Squeeze and apply" glazes are porous, shrink terribly, and have poor adhesion. There is no place for them in your body work and certainly not under your paint!

PRIMER - A very misused term, a true primer's purpose is to provide corrosion protection when applied against bare metal, is non-filling, and non-sanding. It provides a foundation for subsequent topcoats to chemically bond to. Most epoxy primers do not contain isocyanates. While not truly a two component (2K) product, it does use an activator and relies on different chemistry to bond to the metal surface. If the primer doesn't require an activator, only reducer, then it is most likely a lacquer based primer. Other types of undercoats commonly called primers are etch, surfacer, or sealer. See respective entries under those names. NOTE: Lacquer based primers (most spray can primer) are hygroscopic, meaning they absorb water, shrink heavily, and are a poor foundation for a "nice" paint job.

ETCHING PRIMER - Also known as wash prime, or simply as etch prime, it is a specific use product. It contains an acid that, when mixed and sprayed onto bare metal, chemcially etches into the metal surface. The etch prime is zinc rich and performs a sacrificial role if a scratch or chip goes all the way to the metal surface. Most true etch primers are sprayed very thin, generally less than a mil total, and are transparent after application. There are combo versions of this primer that contain the etch portion and a type of primer all in one product. Follow the manufacturer's timelines for topcoating, generally must be done in less than 24 hours.

PRIMER SURFACER - A "primer" that is designed to build film depth for block sanding. The vast majority of surfacers are 2K products containing isocyanates. Think of surfacer as a thin, sprayable finishing filler that must be sanded prior to application of sealer or topcoats. Not to be used over bare metal if total area exceeds four sqare inches.

PRIMER SEALER - A "primer" designed to provide for uniform color holdout and chemical adhesion for the topcoats. It is a non-sanding, non-filling product usually applied just prior to topcoating.

TOOTH - Scratch marks created in metal, body filler, or undercoats, to give filler or subsequent coatings something to bite into. It's a way of increasing the surface area without making it larger. For filler applications, 40 - 50 grit scratch marks will give very good adhesion, 80 grit scratch marks are perfect for finishing filler, and 120 - 180 grit is good for many primers. Follow the manufacturer's recommendations for panel prep, as they may be different between suppliers.

SAND SCRATCH - The marks left by sanding on bare metal, body filler, or sandable primers (otherwise known as surfacers). The deeper the scratch, the more material required to hide it. Very undesireable in the final finish.

SURFACER - A 2K primer intended to be sanded, unlike an epoxy primer, to provide a flat surface for sealer and topcoats. Surfacers are generally not designed to be used over bare metal and provide little to no corrosion protection. See primer surfacer.

FILM BUILD - The thickness of a layer, or layers, of paint or primer. It is usually stated in mils (one mil equals .001"), or microns (one micron is .00004"), and is the basis by which proper sprayable material is applied. The generally rounded figures are 25 microns per mil. For example, PPG DPLF (epoxy primer) is supposed to have a film build of 1.2 - 1.5 mils (or 30 - 38 microns) when applied over bare, untreated metal. Maintaining film build at manufacturer's specifications is critical to product performance.

SINGLE STAGE URETHANE - A two component (2K) polyurethane paint containing pigment and shine in one package. Uses isocyantes in the paint formulation, and provides a durable, long-lasting finish. The clear portion of the paint rises to the top of the film as it dries. Once fully cured, the clear is approximately .5 mil thick.

TWO STAGE URETHANE - Commonly called "Base Coat, Clear Coat" (abbreviated BC/CC) and is a two step painting process. The first stage is the base color followed by a clear coat that provides the shine. This leads to a highly durable finish with very good UV protection because the clear coat is approximately 2 mils thick. Requires the car to be painted twice, once with the base color, and then again with clear.

TRI-COAT AND MUTLI COAT URETHANE - Essentially the same as BC/CC processes except with additional base coats (called mid-coats) to build greater brilliance. May also involve an additional clear coat designed to guard against graffiti. Found on luxury class cars and is usually a high dollar finish (read expensive!!).

CUT AND BUFF - A trade phrase used to describe the process of removing minor paint imperfections by 'nibbing' and buffing to restore shine and depth of image.

LACQUER - A non-catalyzed, single component finish that will dissolve into it's own solvent even after it dries. Dries by solvent evaporation. Rarely used today and increasingly difficult to find at most suppliers. With the advent of modern refinish products, there's no reason to use this for your car. To get the ultimate shine, it required many hours of handrubbing to eliminate texture. Also available as acrylic lacquer.

ENAMEL - A non-catalyzed, single component finish that dries by solvent evaporation. Provided a nice gloss right out of the booth and eliminated the hours of buffing required by lacquer paint. Took as much as 6 months to fully dry. Was produced in alkyd and acrylic formulations. Still available today at many low end shops like Scheib and Macco.

NON-CATALYZED - A non-hardened paint product that dries through solvent evaporation. Non-catalyzed paints do not cure. These paints contain virtually no UV inhibitors and will deteriorate rapidly if not protected. Some later acrylic enamels did use a hardener, which improved durability.

CATALYZED - A hardened paint product that dries by solvent evaporation, and cures through chemical cross-linking. More than 90% of today's auto manufacturer's use a catalyzed, urethane paint at the factory. Most catalyzed urethane paints contain isocyanates.

ISOCYANATES - A chemical used in today's polyurethane paints to create paint film cross-linking. While this makes a polyurethane paint very strong and extremely durable, it exposes the painter and anyone in the general vicinity to extreme health hazards if certain personal protection requirements are not followed. For more information visit this OSHA website -

HVLP/NON-HVLP - High Volume, Low Pressure. Used to describe the type of spray gun process used in applying paint. Using HVLP spray gun requires much greater air volume, but at a lower pressure, than non-HVLP spray guns. An HVLP spray gun must have less than 10psi at the aircap and provide at least 65% transfer efficiency (percent of material released from the gun that reaches the surface). Most HVLP spray guns use about 30 psi at the gun to get 10psi at the aircap. Older non-HVLP spray guns will have significantly more pressure at the air cap, require as much as 60psi, or more, at the input, and have transfer efficiencies of 25 - 35%. Some municipalities require shops to use HVLP guns to meet VOC standards. There is a side benefit however, HVLP spray guns generate much less overspray, so more paint actually reaches the surface. This means less paint is required to achieve the required film build on the surface.

VOC - Volatile Organic Compound, the airborne component of paint lost in the atmosphere. Said to contribute to weakening the ozone layer and increasing greenhouse gases, the EPA is strictly monitoring this in many locations. Some areas in the country, like in Los Angeles, CA for example, body shops are required to use waterborne paint to cut back on VOCs released to the atmosphere. Not usually an issue with a novice or enthusiast who might paint one or two cars a year.

Well, that's a start hope it helps! I'll add to this when I have time. 8)
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