#Bob Story's Automotive Paint Frequently-Asked Questions

For more information about this faq, please contact Bob Story at bob.story at gmail dot com

Please check that your question is not answered before asking me a question. Thanks.

FAQ Revised: Saturday 01 December 2001 13:14:06

Table of Contents

1. Adjusting Paint Gun

1.1. How do you properly adjust a standard spray gun? I'm assuming a conventional suction feed spray gun. An HVLP is a little different. Set your air pressure first. The gauge at the gun should read approximately 55 lbs with the trigger pulled far enough to allow air and not fluid. Point the gun at a piece of masking paper, and pull the trigger a bit further to allow fluid. The pattern should be convex, with slightly more paint in the center than the edges (I'm going to have to put some pictures on my page since this is a commonly asked question). The idea is with a 1/2 overlap, to evenly distribute the material.

2. Aerosol

3. Air Pressure

3.1. I have read your recommendation for a compressor that will put out 20 cfm at 90 pounds of pressure and don't quite understand why such a large compressor since I have so far not found a spray gun that calls for more than 10 cfm. Can you explain the recommendation for a 20cfm compressor more? Theory and practice are like apples and oranges. Does a compressor really put out it's rated delivery, and does a paint gun really require only 10cfm? Or does it meet VOC requirements if you spray at 10cfm? I've been spraying paint for a long time, more than twenty years, so I suppose I'm just asking you to take my word for it.

4. Air-brushing

4.1. Can these same tips be applied to air-brushing? I am trying to air-brush designs on my motorcycle. Please let me know. I generally use acrylic enamel (uncatalyzed and reduced to a very thin viscosity) for air brush work, although I admit I don't do much of it anymore. It might be a good idea to get your base color finished with a urethane clear coat so you have a nice base to work with. After you are happy with your design, clear over the acrylic enamel with the same urethane clear. It sort of goes against the rules, since uncatalyzed acrylic enamel SHOULD wrinkle when clear is applied over it, but maybe because it is so thin it doesn't (or at least it hasn't for me). Lacquers dry too quickly and plug up on the tip of the air brush, and catalyzed paint cannot be stored. Acrylic enamels are cheap, and you'll only need a few basic (bright)colors to tint with blue red yellow black and white to give you all the colors. Oh yes, one last thing, don't buy tinting colors, buy mixed colors. The tinting colors don't have driers added and may not work properly.

5. Antique

5.1. I have a 1928 Model 72 Chrysler that requires painting. I understand the present paint is Duco. Can I obtain this type of black paint? Duco is Duponts old lacquer system. It is very inferior to their current line of paint products, so unless you absolutely insist on original paint, use the newer urethanes.

6. Application

7. Automotive Paint

7.1. WHAT TYPE OF PAINTS ARE USED ON AUTOMOBILES? Japanese Varnish: In the early years, between 1900 and the 1920's, Japanese varnishes were used. The varnish was applied by brush. Nitrocellulose Lacquer: in the 1920's, several paint manufacturers were involved in the development of nitrocellulose lacquers. This paint had rapid drying and low viscosity properties, and was applied with air pressure through a spray gun leaving a hard dry finish in approximately one hour. When rubbed, polished, and waxed, it far surpassed in durability and appearance the qualities of the Japanese lacquers. Synthetic Enamel: In the mid 1930's, a new and completely different type of paint was developed, the alkyd or synthetic enamels. It proved to have superior qualities in film strength, adhesion, luster, flexibility and durability over all previous paints. The resin base was developed from the reaction between phthalic anhydride and glycerin, with gums, oils and plasticizers added during the manufacturing process a drying oil such as linseed, a polyhydric alcohol, such as glycerine, and a dibasic acid, such as Phthalic Anhydride. It dries by solvent evaporation, like the lacquer paints, but the resin remains soft and sticky when no solvents are present. It cures to a hard finish by absorption of oxygen from the air. The curing process can be accelerated by heat, and several methods of baking enamel were developed. Unlike lacquer, when dry, it needs no polishing to produce a high luster finish. Acrylic Lacquer: As time passed, chemists developed a substitute for nitrocellulose lacquer, using an acrylic resin as a base. The resins used in acrylic lacquer tend to be slightly brittle. This deficiency is overcome by the use of a plasticizer ( a liquid that is a solvent for these resins and softens them slightly). A cellulosic resin is any resin derived from cellulose (pure cotton). Acrylic lacquer was used extensively by General Motors. Acrylic Enamel: During the late 1960's and early 1970's, technology brought on the development of acrylic enamel, which was harder and more durable. Chemically, it is a cousin to synthetic enamel, but is modified with acrylic resin, and is not soft and sticky with no solvents present. It cures further with the absorption of oxygen from the air. Unlike the lacquers, which remain soluble in solvents, the enamel family is insoluble in solvent when cured. An acrylic resin is chemically any polymer whose basic monomers are chemical derivatives of acrylic acid. Polyurethane Enamel: In the mid 1970's, polyurethane enamel was developed to withstand the severe stress of high speed airplane surfaces, which are subject to rapid temperature changes and flexing. This paint was much more durable than the acrylic enamels. Acrylic Urethane Enamel: Acrylic urethane enamels were developed to withstand environmental elements, such as acid rain and ultra violet rays. It is the most durable paint to date.

8. Bare Metal

8.3. WHAT DO I APPLY OVER BARE METAL? Use a zinc chromate primer which is essentially a metal treatment. Follow this with a good quality primer surfacer (a brand name that requires a catalyst (urethane or epoxy)). All the major manufacturers use a form of a seven stage process before priming.1. Hot detergent wash to remove grease and oils. 2. Water rinse to remove the detergent, since the detergent interferes with acid. 3. Acid wash to remove scale and open pores in metal. 4. Water rinse to remove acid. 5. Zinc phosphate acid treatment for corrosion protection and paint adhesion. 6. Water rinse to remove acid. 7. Bake dry.This is followed by a primer coat of melamine polyester resin (Japan), polybutadiene (Europe), or a polyester or epoxy ester system (USA).

9. Base Coat Re-coat Window

9.1. After spraying a base coat do I really only have 30-45 min. to spray the clear coat? No, you can apply the clear much later if you like. There seems to be a difference of opinion on the maximum time frame; I've heard anything from 72 hours to infinity. I asked the Dupont techs on their tech line, and they don't seem to know either, and it isn't listed in the technical manual. I've done it three days later with no apparent ill effects.

10. Base Coat Sand-ability

10.1. How come you can't sand a base coat in a BC/CC combination if there are imperfections? You can. I usually go over the base coat quite carefully before I clear, and fix any imperfections. You should use very fine paper.

11. Bird Droppings

14. Brain damage

15. Brand Names

15.1. WHAT BRAND OF PAINT SHOULD I USE FOR MY FIRST ATTEMPT AT SPRAY PAINTING? All major brands of paint perform well and it is not my intent to recommend any one particular brand. However, certain paints are better for the first time painter, although they may have inferior qualities in other areas. A novice or first time painter might try Dupont acrylic enamel (Centari (tm)). It is one of the most forgiving paints, as it dries quickly, melts in over-spray well, and does not run easily. These positive features outweigh the negative effects of quick dry times, which are mentioned elsewhere in this FAQ. I don't know if they still make it, but Deltron(tm) by PPG is also an excellent product for a first time attempt.

16. Brush Painting

16.1. I've read that automotive paint can also be applied with a brush and with good results. Do you have any experience with this approach. Can it possibly yield OK results ? No. A professional spray gun is an expensive precision tool. With today's mica's and pearls, the finish has to be finer than 320 grit, or the particles will not orient themselves correctly. The paint gun has to provide a fine, even mist to enable the correct transfer of material which may be less than 1 mil (one thousands of an inch) thick for some base coats. As for applying these coatings correctly with a paint brush there would be a better chance that a random collection of atoms in outer space could gather together to form a Nike running shoe.

17. Budget work

17.1. I know this is kind of like blasphemy within the professional auto painting industry, but what do you think of Maaco's jobs? I know you all say "you get what you pay for", was wondering if you could qualify this with respect to Maaco. I have never dealt with them, but I'm sure they provide a good budget service. Most shop rates here are close to $50.00 an hour, so divide that by the cost and you get how much time they can spend on your car and still show a profit. No one should expect a complete restoration for a budget price.