Getting it perfectly straight will always require some filler. Fillers are mixed with the hardener in the prescribed ratio, and applied with a metal or plastic squeegee. The key is to lay the filler were it is needed, without excessive thickness where it's not. This '57 Chevy quarter, being worked on by D & P Classics in Huntington Beach, CA, will be glass-smooth when complete.
Fill It
Body filler is one of the most useful, and at the same time, most abused substance in the automotive hobby. Try as you might to get the metal perfect, only the very masters of the metalworking craft can metal-finish a panel so that no filler is required. Some filler will always be required if you are looking to achieve pin-straight bodywork, and even some areas of the factory panel work are not straight enough for show-quality paint. If major panels have been replaced, some filler use is virtually guaranteed. Modern plastic fillers, applied properly, will outlast most paint-jobs. Filler should be thought of as thick primer, to be used in a light skim to build material-not as a crutch for substandard metal work.
Filler is a polyester-based paste that begins to cure when mixed with hardener in the ratio prescribed by the manufacturer. The filler will remain workable only for a few minutes, and is applied with either flexible plastic or metal spreaders. Spread the filler smoothly, feathering off well past the actual repair area. Once the filler has taken a partial set, some pros like to file it with a Surform blade to form the gross shape, in a process called cheese-grating. We find the half-round blade to be the most useful profile, though when working in thin sections, especially if the plastic has been nicely applied, cheese-grating will usually be unnecessary, and can actually do more harm than good.
Filler is usually sanded with a long, hard sanding board, or an air-operated inline sander. Use a criss-cross shallow diagonal motion to level the filler. Start with #60 grit paper, or #36 if lots of material needs removal. When the surface is level, sand out the deep sanding scratches with #80-100 grit paper. Any low spots will show up as shiny areas the sandpaper didn't reach during the coarse sanding, indicating the need to reapply filler as required. Finish sand so that the filler meets metal with smoothly feathered-in edges.
The relatively course texture of plastic filler will often show pinholes and other surface flaws. Glazing putty is usually used over the filler, which has a much finer grain and fills surface flaws, while giving a finer feather-edge. Glazing putty comes as a simple single-component fine-grained acrylic putty, or as a two-component putty with a hardener. These days, two-component spot and glazing putties are preferred due to less shrinkage and a quicker cure than older acrylic air-dry putties. Glazing putty is sanded similarly to the base filler after it cures, but use finer paper, finishing with #150, or finer.
The quality of the finished product will largely depend on the sweat produced here, blocking the sheetmetal to perfection. The long sanding board will knock down the high spots, and leave primer build-up in the low spots. This primer and blocking process is repeated until perfection is achieved.
Block It
The primer and blocking stage of any paint job is what separates the men from the boys, and the show cars from the fun field. Here, we are talking about the tedious handwork that results in the unbelievably straight and smooth look that defines top-quality paint. Before starting the build-up and blocking with primer surfacer (also referred to as primer filler), the bare metal is best sealed with an epoxy primer. We believe epoxy primer offers the best protection, durability, and corrosion resistance. The primer-surfacer comes next, and is applied for the final leveling and smoothing of the body panels. Surfacers are formulated to build up relatively thick, and are easy and smooth-sanding.
Surfacer is applied to level minor flaws, and provides a very smooth and level final surface for paint. If applying surfacer over cured epoxy, consult the manufacturer's rep for the correct surface preparation procedure for the products being used. Typically, cured epoxy will need to be sanded or scuffed for adhesion. Before block sanding the surfacer, a mist coat of a contrasting color should be sprayed over the primer to serve as a guidecoat. The guidecoat will remain in low spots after sanding, indicting where further attention is required. Very minor spots may be filled with a second primer coat, though other areas may need a skim of spot or glazing putty.
Primer surfacer imparts a more subtle filling effect, leveling minute variations in the sheetmetal that will be readily picked-up by the eye once the final gloss is applied. (Don't call us! Always use proper a respirator when painting. This shot was mocked up.)
When blocking the surfacer, we like to use a fairly aggressive cut on the first round of sanding, cutting with a hardboard and #150-180 grit dry, or #220 wet paper. The more aggressive grit will give the greatest leveling effect. When sanding, follow the guidecoat to make sure the panel is thoroughly sanded. If the guidecoat is still showing in an area where the adjacent surfacer is being cut completely through, more fill is required. For a perfect result, the primer and block sanding process may need to be repeated several times-the guidecoat will tell you when perfection is achieved. A finer paper is used for successive rounds of block sanding, usually going to #600 grit wet for the final sanding. Some surfacers are a little too soft for topcoating, and require a sealer coat before final paint. Follow the manufacturer's recommendations for the product systems used.
Automotive paints come in a bewildering range of formulas, and most any color can be created by a combination of base colors in a full tinting system. This Deltron system from PPG is used by D & P Classics (Huntington Beach, CA), but your local paint jobber supply should have everything you need if doing it yourself.
Shoot It
After all of the preparations involved in a high-quality paint job, it comes down to the moment of truth-shooting it. Here, the quality of the prep work will show its worth, or lack thereof. When everything goes right, the actual shooting of the car can be one of the most satisfying parts of an automotive project. For a clean paint job, the finish coats must be applied in a proper paint spray booth, which should be meticulously clean for the job. Care in the final preparations are the key to success, including detailed final masking, as required, which often involves masking the previously painted jamb areas, engine bay, or lower perimeter to protect the chassis. A wipe down with wax and grease remover will insure a clean, film-free surface, followed by several rounds with a tack-cloth to ensure the sheetmetal is spotless.
There are a variety of materials used for topcoating, but the most common today are acrylic enamels and urethanes. These catalyzed paints are very durable, and provide exceptional gloss. There are two distinct systems typical of modern paints, single-stage, or two-stage. A single-stage paint job is simply a single paint product that is sprayed on to provide both the color and the gloss. Two-stage paint jobs, also known as basecoat/clearcoat, begin with the base, which imparts the color, but dries to a matte finish, requiring subsequent clearcoating to provide the gloss. Either system can provide excellent results, but two-stage paint jobs are more typical of custom work, since this technique lends itself to a final cut-and-polish, especially with metallic or custom finishes.
Mixing the paint is a critical step in the process, and each manufacture gives specific guidelines on mixing ratios of the various paint components of reducers and hardeners. Typically, the manufacturer will have several products, such as reducers, in the line that must be selected based on working conditions such as the ambient temperature. Some painters will deviate from the recommendations, looking to modify the formulas for enhanced flow-out, or self-smoothing as the finish is applied, particularly in areas where the manufacturer is forced to limit the solvent content to meet the regional VOC (Volatile Organic Compound) emissions requirements. Experiment at your own risk.
Equipment is another key consideration, and here, quality counts. Modern paint formulas are very sensitive to the flow-rate, atomization, and pattern of the spray equipment. Don't expect to achieve show-car results with a $20 gun from the local farm supply. A gun from a top quality manufacturer will save a world of frustration and regret. While historically spray equipment worked off a siphon-feed system, most of today's guns are gravity-fed HVLP (high-volume, low-pressure) units, which considerably reduce paint overspray and waste. The actual application technique is a learned skill, but the basics are to hold the gun perpendicular to the surface, sweeping in before pulling the trigger on a pass, and following through after the release, while holding the gun a consistent distance of 10 to 12 inches from the surface.
Various tinting bases are metered out according to the prescribed formula to mix the desired hue. A special scale reads the amount of each ingredient in the correct proportions. | 
Before the paint is loaded into the gun, any catalyst or reducer must be added as prescribed by the manufacture's specifications. There are normally different grades of reducer based upon flash time, or even different catalysts to be used for varied shop conditions. | 
Taping is important, since wayward paint overspray can cause serious damage to areas of the vehicle where it shouldn't go. The inner jambs of this Camaro were painted before the overall refinish, so they were carefully masked to prevent overspray from ruining the job. Note that the tape is folded over itself at the corner's edge. This soft "back-masking" technique will minimize any raised paint edge at the tape line. |
Buff It
While a basic paint job is complete with the last pass of the paint gun, most high-quality or show finishes are cut and polished once cured. This involves color sanding the clearcoat, and then using a paint buffer with a sequence of pads and compounds to bring the surface to a mirror-like gloss. Sanding the surface will remove tiny flaws and spray texture, giving the glass-smooth look of perfection. Sanding is done wet, with very fine grit sandpaper, with the actual grade depending upon the amount of leveling required. If despite your best efforts, the clear has a moderate level of "peel," or texture, more aggressive grit will usually be required. Normally, #1000 grit paper is coarse enough for heavy cutting, but often #1500 or finer will do the job. A major factor is to be careful not to cut all the way through the clear during the sanding process. Following-up with a finer paper, such as #2000 grit, will reduce the depth of the sanding scratches, and make the polishing to follow much easier.
Wet-sanding is complete once the surface is evenly dull and smooth. Any shiny spots or gloss left are areas that require more sanding for a perfectly level surface. Care must be taking in the process, since cutting all the way through the clear would be a disaster.
Buffing is done with a low-speed (1,800 - 2,200 rpm) rotary buffer specifically designed for automotive refinishing. Minimally, buffing is a two-step process, starting with "cutting" compound, and a wool "cutting" bonnet, followed by the polishing compound or glaze, applied with a finer wool or foam polishing pad. There are numerous manufactures of compounds for cutting and polishing paint, and it pays to stick with a system of products from a single supplier. The final polish can make a mediocre paint job pop, and will give a nicely applied topcoat a brilliant look, but it will probably make the overall effect worse with shoddy prep-work and wavy panels. Great paint requires considerable skill and knowledge, and it all comes down to making the effort for a dazzling final result.