Automotive Refinishing: After the Paint Dries
By Matt Joseph Most of our experience is, or course, with dried or cured paint. Refinishing an old car is a complex, time-consuming and expensive proposition, and most of our…
By Matt Joseph
Most of our experience is, or course, with dried or cured paint. Refinishing an old car is a complex, time-consuming and expensive proposition, and most of our efforts are necessarily directed to making existing paint work. Sometimes this is paint that we have recently sprayed and that needs further work to produce an acceptable finish. At other times, it is possible to work with an older, existing finish in a way that allows us to improve its appearance to the point that it becomes acceptable.
Occasionally, the problems of an existing finish or of a new finish are so great that the only practical approach is to refinish. All of these situations differ from those discussed in the previous two chapters because they involve existing finishes. Finally, after a finish has dried, it must be maintained and sometimes repaired if it is to have a reasonable service life.
Compounding and Polishing
Old, oxidized or stained finishes and new, unleveled rough ones require polishing to achieve a smooth and attractive luster. Compounding and polishing are essentially the same operation, except that compounding involves coarser abrasives. Both compounding and polishing level painted surfaces by abrading or “scratching” them very minutely. While it may sound strange to consider scratching a surface to polish it, this is the nugget of virtually all polishing processes. The key is in the size of the abrasives that are used and the sizes of the scratches they impart. Just as 300- or 400-grit abrasives will visibly scratch and dull a finish, abrasives in the 1500-grit and above range will polish it. In the latter case, the scratches are so fine and closely spaced that they produce the optical effect of gloss and luster. That is, until you look at them under a microscope; then they look like what they are—scratches!
Compounding is the first step, and polishing is the second step in bringing full luster to a rough finish. Finishes that are already pretty level do not need to be compounded, and you can go directly to polishing. Both of these processes can be accomplished either by hand or with a machine. Hand compounding and polishing have the advantages of vastly reducing the risk of cutting through a finish on edges or at styling reliefs. Hand compounding and polishing often produce fewer objectionable swirl marks than machine compounding and polishing because they can be performed in relatively straight lines, as opposed to the circular motions characteristic of machine polishing.
The main problem that some people have with hand compounding is that it represents a variant of the human experience known as “WORK,” and even “hard, dull WORK.” Machine compounding and polishing are also “work,” but never “WORK.” While I prefer the appearance of a hand-compounded and polished finish, the amount of effort required to level and polish lacquer or catalyzed enamel finishes by hand can be considerable in some cases. It all depends on what you start with.
Not all finishes can be compounded or even polished. Lacquers and acrylic lacquers dry in a way that invariably leaves a surface that will be improved by polishing and/or compounding and polishing. Catalyzed urethane and cured or hardened alkyd enamels can be polished with fine abrasives, and this should give them an added gloss. Alkyd enamels cannot be polished until they have aged for six months or a year.
I consider polishing any kind of enamel to be an unnecessary and inappropriate step in restoration work. This is, of course, a personal opinion and goes against a lot of modern restoration practice. I think that if these enamels are properly applied in an appropriately dust-free environment, they will achieve a natural gloss that I find far more attractive and authentic than the one which includes the swirl marks that accompany polishing.
The abrasives used in compound and polish are made from anything from Tripoli to pumice, talc and synthetic abrasives. Hand polishes and compounds generally use oils for their bases, while machine compounds and polishes are usually water-based. You can get compounds and polishes in different grades, depending on how aggressively you need to remove surface films and decayed paint to clean and/or level a finish, and how fine you want the abrasives for your final polishing to be. As in any abrasive finishing operation, you move to progressively finer grits to produce glossier and shinier results. When working with enamels, you begin with finer abrasives than those used initially on lacquers. Both machine and hand abrasives are designed to break up into ever finer particles as polishing with them progresses and the final stages of shine are achieved. Abrasives designed for machine application tend to break up faster than those designed for hand application. Machine compounds also tend to dry (evaporate their solvents) faster than hand compounds because of their different bases. In general, use hand abrasives for hand compounding and polishing and machine abrasives when you use a machine for this work.
It should also be noted that there is a tremendous difference in the quality of various brands of compounds and polishes. The ones sold in body shop supply stores and marked “for professional use” invariably produce better results than the cut-rate stuff sold in discount stores. The professional stuff doesn’t come in neat little $1.29 tins, so if you want good quality compounds and polishes, be prepared to spend more than that. In working with polishes and compounds, employ the least abrasion and surface removal that will get the job done. In the case of old finishes with deep scratches, or of new ones with excessive orange peel, a fairly course grade of compound or even a preliminary “color sanding” with a 600-grit abrasive paper, followed by smoothing with 1200-, 1500-, 1800- or even 2200-grit abrasive paper, may be necessary for the initial leveling. Developments in coated abrasive technology have produced papers that are much finer than those that were available a decade ago. Some of these are consistently higher than the old 1200-grit papers, and are not just made up of particles that average to a stated grade. While these new papers can be used to do terrific leveling work, they should only be used when it is absolutely necessary to remove quite a bit of material for leveling purposes. They also tend to be quite expensive.
One unfortunate tendency that has crept into the practice of some body shops, and even of some restoration shops, is the business of applying finishes, particularly catalyzed enamels, badly and in grossly contaminated environments. This dirty application is then routinely followed by color sanding to level and clean up the finish, and then by machine compounding and polishing to give it gloss. This approach produces superficially attractive results, but close inspection will indicate the sins of unevenness and contamination in the original paint. The options of color sanding and wheeling a finish can be valuable, but they should never become a routine part of refinishing with any material, particularly catalyzed enamel.
It is also important to understand that, just as some finishes, such as uncatalyzed acrylic enamel, cannot really be effectively polished right after they are applied, there are other limits to what can be done after the fact with abrasives and finishes. Acrylic lacquers, for example, require some form of polishing after application, but if you attempt to do this too soon, you will dull and ruin the surface on which you are working. Sometimes, older finishes that have chalked (oxidized) can be saved with compounds and/or polishes, but other times they are deteriorated to the point that they can no longer be polished.
Whether a hand or machine application of compound or polish is being attempted, it is important to mix the compound thoroughly before you begin using it. In hand compounding and polishing operations, the abrasive is applied to a soft damp pad or wad of rag and the surface is rubbed in straight lines. Apply only enough compound to do the area that you are working on, and confine this area to about the size of a car door, or less. As the surface begins to polish and the abrasive breaks up, your rubbing strokes will encounter less resistance; you will feel this happen. At this point, you can ease up the pressure on your applicator. Finally, the mostly dried compound should be buffed with a clean cloth and very little pressure. A second clean cloth can be used to advantage for a final buff.
While it is much more difficult to cut through a finish in a hand operation than in a machine operation, it is possible. This is particularly true when you employ the rougher grades of compound. Course compound is also a hazard when it is used on older finishes where you really don’t know how much color coat there is left to compound or polish.
When you compound or polish, be particularly careful of raised edges, crease lines and other places where a panel bends sharply. Not only does the pressure from hand, and particularly from machine, polishing tend to concentrate in such places, but the finish is thinner there to begin with, and consequently, it is easier to cut through it. Be careful.
Machine compounding and polishing are much faster than comparable hand operations. A tufted wheel or a “bonnet” is used with paste type abrasives, and a lamb’s wool bonnet is used with liquid polishes or with very fine paste type abrasives. There are several other new polishing head configurations on the market. Effective polishing machines operate at around 2,000 RPM, or less, so you should avoid using the faster hand-held machines that are really designed for grinding. Excessive speed or pressure will either cut through paint, or burn it. Good wheel polishing is done with only moderate pressure, and you must keep the wheel moving at all times to avoid burning or cutting through a finish. As you polish, your bonnet will tend to clog with paint and spent abrasive, and it must be cleaned periodically by running a cleaning tool over it to remove the clogs.
In machine polishing, abrasive is applied to a surface that is being polished and the machine is moved first horizontally back and forth with overlapping strokes over a small area.
The machine is then moved vertically up and down with overlapping strokes over the same area. The edge of the pad or bonnet should be lifted slightly, about 1/2 inch, in the direction that the wheel is being moved, either horizontally or vertically. Do not continue to polish after the liquid lubricant in your abrasive has evaporated or you will damage the finish. Also, do not attempt to work too large an area at one time. A door or fender will be about right.
Machine compounding and polishing can produce dramatically favorable results, but it is also very possible to produce burns, scorches and cut-throughs with surprising ease. Edges and bends in panels exacerbate these problems, and such areas should be avoided when you wheel panels. If it is difficult to avoid these areas with your wheel, it is best to tape them with masking tape to prevent problems. After machine polishing is complete, you can remove the tape and do the vulnerable areas by hand. It is almost never possible to get safely into every painted surface on a car with a wheel, so some hand work is generally necessary. Caution is the byword here, because it is a terrible feeling to have to repair a new finish that has been damaged by cutting through it, or burning its surface, with a polishing wheel.
It should also be noted that polishing wheels throw off fine airborne abrasives in considerable quantity and with great velocity. It is necessary to wear goggles and a respirator or dust shield when operating one of these machines.
Spot Repairs to Damaged Finishes
The ability to spot repair a finish is often utilized in restoration practice when damage is confined to one, or to a few, small areas. The objectives of spot repair are to produce a finish that blends into the panel being repaired and that matches it in gloss, color, texture and level. While spot repairs can be made with finishes other than acrylic lacquer, this is certainly the easiest modern finish to work with in this way, and it usually produces very acceptable results in spot refinishing.
Spot refinishing generally follows the outline given for panel and overall refinishing, except, of course, that there is the additional problem of blending the edges of the repair area into the old finish. The first step in spotting in work is to wash the panel where the repair will be made. Do this well beyond the edges of the repair area. Wash with water and mild detergent, and then rinse thoroughly with clean water. This should be followed by a solvent wash with a silicone and oil removing solvent. The repair area should then be sanded down as far as is necessary.
If a paint defect was the reason for the repair, it must be sanded to bare metal or to the primer, if it is still intact. If there is rust, or if metal repair was necessary, the sanding must go to bare metal in the repair area. When bare metal is exposed, it should be treated with metal conditioner. Then, the excess conditioner must be wiped off the surrounding finish with a damp cloth. If you are simply repairing a scratch or similar superficial defect, you should sand far enough to prevent excessive film thickness when the color coats are applied. The total thickness of paint and primer should not exceed 8 or 9 mils, in any case. There are various devices for measuring paint thickness, such as the magnetic Tinsley Gauges or some very fancy electronic (ultrasonic) gear. Generally, with some experience, you can tell by sight and feel how much paint is on a panel when you sand through it to bare metal in one spot. Come to think of it, those paint thickness gauges are not a bad idea.
Whether bare metal or primer remains after sanding, the edge of any repair area must be feather sanded so that it gradually comes up to the level of the areas beyond the repair area. There are wiping “lacquer dissolving” solvents available to feather lacquer finished surfaces for this purpose, but these tend to produce an inferior result to hand sanding feather edges and should only be used on very small spots, if at all.
Surfaces well beyond a feather edge sanded area should be masked to avoid deposits of overspray. If there is bare metal in the repair area, it should be primed. Then, a primer surfacer with good holdout characteristics should be used to fill and level the entire repair area. The technique of feathering a spray gun—releasing the trigger partially and applying less paint on the feathered edges of a repair area—can be utilized to great advantage in this work. Be sure to allow adequate flash times for primer-surfacers and for the color coats that come later. Remember, drying paint shrinks roughly 50 percent from its wet state.
Some refinishing practitioners advocate compounding the feathered edges of a repair area before spraying primer-surfacer. Hand compounding is best if this procedure is followed, but machine compounding can be used and is advocated by some painters. I have never found compounding necessary at this point, but prefer to do careful sanding with uncontaminated 600-grit paper.
In either case, the area should be cleaned with water and then with solvent prior to applying primer-surfacer to it. It is generally recommended that a cleaning solvent be used for this purpose, but I have found that these solvents tend to soften the primer and can produce several problems later. A fast enamel reducer is almost ideal for this kind of solvent cleanup operation. After the primer-surfacer that was applied to the repair area has dried adequately, the area should be block sanded or board sanded to level the repair area with the surrounding panel surface. This area should be cleaned with a solvent cleaner or fast enamel reducer after it has been washed down with water. You are now ready to apply color coats.
If the repair is to be made with acrylic lacquer, a two-gun technique will serve best. The basic proposition here is to cover the repair area with as many color coats as are necessary to achieve hiding of the primer and to blend the edges of the repaired area into the existing finish. When hiding is achieved, a mist coat of highly thinned color (5 percent) is applied with the second gun to the edges of the repair area. This mist coat will blend those edges into the surrounding finish. A product like DuPont’s Uniforming Finish is ideal for this purpose, but, of course, this should only be used with DuPont systems. Other manufacturers have special products or special procedures with conventional thinners and reducers to accomplish the same result. While it is generally effective to follow color coating with a blender coat, sometimes more than one blender coat is necessary, and sometimes it is necessary to alternate color coats with blender coats around the edges of spotted-in paint after applying a second color coat.
It is much more difficult to effect spot repairs with enamels, but it is possible, and with some finishes it is necessary to do this to get a texture and gloss match. Enamels are used in roughly the same way that lacquers are for spotting in, except that straight retarder is usually the solvent used in the blender (mist) coat. The blender coat is sprayed over the edges of the repair and it is sprayed very dry, and only after the color coats, never between them. The blending process with enamels is almost a fog coating process. When the repaired finish is thoroughly dry, it should be compounded well beyond the repair area to remove any overspray and to blend it into adjacent areas that were masked during the repair. If everything was done properly, a repair can be made that is indistinguishable from its surroundings.
Remember, body shops accomplish such repairs with metallic colors every day, and compared to the problems encountered in that proposition, spotting in solid colors can be kid’s stuff.
You will note that I have not discussed how to effect spot repairs in clearcoat/basecoat systems. That is because I do not know how to do this and doubt that it can be done with results that are acceptable for restoration work. I have talked to about a dozen painters who say that they can do it without leaving the telltale dull ring around the repair area in the clearcoat. But I have not yet seen anyone accomplish it.
There are some points that will help you to make successful spot repairs. The first is that this is demanding work, and it is important to strenuously observe all of the general cautions of good paint practice regarding cleanliness, paint mixing, solvent choice, gun technique and the like. It is also important to choose the boundaries of your spot repair area carefully so that the contours of panels will work to your advantage and not against you. If, for example, your chosen boundaries end at panel edges, it will be much more difficult to make a repair that does not show.
It is also important to remember that while it is possible, by rigorous attention to detail, to control the leveling of a repair area, its gloss and the quality of its blend into surrounding areas, you will have great difficulty controlling its color match. Good paint mixing and careful attention to gun technique will help you achieve a good color match, but if the repair paint does not match the surrounding panel, there is very little adjustment possible with gun technique alone that will save the situation. The use of a factory tinting kit may help.
Another useful technique is to raise your masking barrier and check the color match after you have sprayed two or three color coats. Color adjustment can be made at this point with the final color coats, as necessary.
One of the surest ways to ruin a spot repair is to neglect to provide adequate flash times with primer and color coats as you go along. This factor should be watched carefully.
Very Minor (Brush) Repairs
Automobile manufacturers sell and people buy little bottles of “repair” or “touch-up” paint that allegedly can be used to repair modern automotive finishes. This has been going on for at least 60 years. Business must be pretty brisk because all kinds of aftermarket manufacturers also sell little bottles of this stuff. Actually, there isn’t anything particularly wrong with what is in the bottles, but it is what is not printed on the bottles that can get you into trouble. Remember that these are small bottles, about the size of a Magic Marker, and there isn’t room to print much on their sides. Generally, the instructions suggest that it is important to shake the bottle until the agitator ball can be heard, and for some discreet period after that.
Then, the instructions often seem to trail off into vagueness and only generally hint at application technique.
Armed with these bottles, people sally forth and attempt to cover little rust spots and nicks with the handy-dandy little nail polish-type brushes that are included in the lids of the little bottles. You see people performing this repair ritual in their driveways with every expectation that they have preserved the finishes of their cars. Often, the touch-up paint is applied over rust and even over the moisture that is left from washing the cars. The prime time for such repairs is in the fall to protect finishes from the ravages of on-coming winter.
Of course, rust just continues to fester under this kind of ill-conceived repair. Apparently, the well-meaning people who indulge in this fix-up-fantasy don’t realize that the thumb nail-sized bubbles in the paint on their cars often originate in tiny nicks that were repaired this way. They just go on buying the bottles of touch-up paint and performing their touch-up rituals every year.
It is possible to make quick and reasonably effective brush repairs to small (1/4 inch in diameter or less) breaches in finishes, but it takes more than a dab of paint out of a bottle to do this.
Proper brush repairs can be an effective temporary repair technique. To make them durable and of satisfactory appearance, you still must follow the logic and sequences of proper refinishing. This begins with removing all rust from the area to be repaired.
A dental pick can be very handy for this purpose, particularly where rust has gotten into the pores of metal and produced pitting. Spot blasters also work well for this application. The repair area should then be sanded and feather edged. Since the object of brush repairs is to keep the repair area small, the feathering can be done with a lacquer dissolving feather edging solvent if lacquer is the finish being applied. Otherwise, you can rotate a piece of 320- or 400-grit sandpaper under the tip of your finger to produce shiny metal and a feather edge at the repair site. The bare metal should then be treated with metal conditioner and the excess wiped off the surrounding paint with a damp cloth. Next, the repair area should be primed and filled by lightly brushing in coats of primer-surfacer until it is roughly level with the rest of the finish.
The primer-surfacer can be applied with either a small artist’s brush or a striping brush. It can also be air brushed through a 1/8-inch round aperture in a 3 x 5-inch card. The air brush and card technique produces the best repair, but you have to vibrate the card mask slightly as you shoot to blur the edges of the spray. This can be something like patting your head and rubbing your tummy at the same time; some people can do it naturally and other people have trouble with it.
Whatever technique is used to apply the primer-surfacer, it should be allowed to dry thoroughly between coats. The characteristic shrink of primer-surfacer, 50 percent from a wet condition to a dry one, should leave room for color coats and keep them roughly level with the panel when they dry.
When the primer-surfacer has flashed and dried, it should be sanded lightly with 600-grit sandpaper. This type of repair will rarely produce perfect leveling and is really a stopgap measure to arrest corrosion and further deterioration of a finish and to produce a reasonably attractive surface. If you try to achieve a perfect level, you can easily sand through the primer-surfacer, so settle for a reasonably level surface with a little depth left for filling with the color coats.
The next step is to brush the color onto the repair in about four coats. It should then be allowed to dry thoroughly. Then the repair area is compounded to blend it into the surrounding paint. Acrylic lacquer works best for this type of repair and should be mixed relatively thin with about 5 to 10 percent retarder included to help it flow out. Don’t try to brush more paint in when preceding coats are tacky and stringy.
While brush repair is far from a perfect repair technique, it is a reasonable maintenance measure for finishes that would otherwise deteriorate from stone chips and other paint film defects. It has the integrity to confront corrosion, and if this kind of repair is carefully leveled, it will have a reasonably attractive appearance.
