Arc & spot welding
Automotive hand tools
Car enamel colors
Car paint colors
Car paint glossary
Car painting problems
Cleaning car upholstery
Infra-red paint drying
Interior automotive trim
Painting over paint
Paint surface preparation
Refinishing paint tips
Safe car spraying
Sanding, striping, rubbing
Shrinking sheet metal
Not finished yet:
Whatever the make of gun you choose, remember this: it is a fine instrument. Treat it as such and it will give you fine service. Neglect it, and it will cause no end of trouble. And the main matter to watch in taking care of any gun is to keep it clean. Also, put a drop of light oil on the moving parts every week. This simple service will make the gun last much longer than it would if you failed to give it this slight attention.
When you finish spraying for the day—or change colours —put thinner in the cup and work the trigger until the spray coming from the gun is entirely free of colour. Some operators make it a practice to soak their guns in thinner. That is not the best method.
The thinner tends to destroy packing in the gun. Furthermore, it is an unnecessary fire hazard. And there is no need of soaking the gun if thinner is shot through it while the lacquer is still soft. Should a pressure tank and hose be used, all paint should be blown from the hose and then thinner should be run through it, by working the gun, until there is no trace of colour in the mist coming from the gun.
Before starting to spray, be sure that you have the air pressure recommended for the gun you have and the material you are using. Gun manufacturers specify the pressure to use.
Insufficient pressure may result in poor breaking up of the material : excessive pressure will cause loss of material through "dusting." Either too much or too little pressure will cause an unsatisfactory finish. The "orange peel" effect is commonly the result of too much pressure, though insufficient thinner might be the cause. A pressure between 55 and 75 pounds is approximately correct, but the exact figure will depend on the gun and the material you are using.
There are important differences between handling lacquer and handling synthetic enamel. Lacquer is more fluid and more volatile than synthetic enamel. Consequently lacquer is sprayed with an air pressure around 45 to 50 pounds, while 60 to 70 pounds is generally needed for synthetic enamel.
Due to the volatility of lacquer, the gun is held closer to the surface—a distance of 6 or 8 inches being preferred. With synthetic enamel it is best to keep the gun 8 to 12 inches from the surface. Working too close with either finish is likely to cause a wet, heavy area that may sag, though the hazard is greater when working with synthetic enamel.
The drying is very rapid with lacquer, so holding the gun too far back will cause the material to dry in the air, resulting in a rough surface. There is less tendency for synthetic enamel to do this.
The action of either product is influenced by the manner of thinning, the lacquers usually being thinned with 1 1/2 to 2 units of thinner for each unit of lacquer. With synthetic enamels there is generally 1 unit of thinner to 5 units of enamel. It is interesting to note that since there is more solid material in the enamel, one enamel coat is equivalent to several coats of lacquer.
Commonly a lacquer finish consists of 3 or 4 double coats. A double coat is generally considered to be the result of covering the surface first with overlapping horizontal strokes and then with a series of overlapping vertical strokes. Studying the spray pattern will determine how much to overlap. Very commonly the spray pattern can be thought of as being in three sections : The centre one-third if correct will cause the outer thirds to thin. And if, therefore, the strokes are overlapped by this amount, the entire coat will be of uniform thickness.
In handling polychromatic finishes, that is those that have the metallic powder in them, lacquers should be applied with one fairly heavy wet coat followed up by two or three mist coats. This is just the opposite of applying the straight lacquer finishes, which should be put on with a thin coat first followed by wetter coats.
In handling the polychromatic synthetic enamels, there should first be a heavy coat sprayed on—being very careful not to get it so heavy as to cause it to sag, and then this should be followed by a thin coat.
The manner in which these two finishes dry differs widely. Lacquers dry by evaporation, and can be polished under normal conditions in about 1 hour. Synthetic enamels have a preliminary drying by evaporation and then harden through oxidation of the oil. No polishing should be attempted for 48 hours unless the job has been oven dried. And even then, sanding or polishing tends to remove and ruin the sealing effect of the synthetic enamel and thereby reduce its durability.
In using a spray gun or in applying a finish, always remember that the manufacturer can be no more successful in selling his product than you are in using it. Accordingly, he always gives the latest and very best advice. In purchasing a new gun, search for the instruction slip that accompanies it. In buying a container of paint or thinner, observe the instructions. They are the best information that you can get.
In thinning any material a little experimenting will be in order. There is a difference in thinners and only by experience can you know the exact quantity to use. This is especially true when spraying heavy material to obtain a glossy surface. Under such circumstances it will probably be necessary to increase the air pressure.
And the greater flow of air accelerates the evaporation of the solvents. This lets the material strike the surface in a somewhat gummy condition with the result that it does not flow out smooth but becomes mottled. More thinner is one way of curing this difficulty. Higher quality thinner, rich in slow-drying solvent is probably the better—and no more expensive—way of getting the desired effect.
So the quality of the thinner you are using does make a difference in the quantity needed for any specific application. Test mixes are worth while. Watching the proportion, prepare a small quantity of material and try it on some old part. If it is difficult to apply, gives an uneven effect, add more thinner. If it fails to cover the surface or tends to be iridescent—has "rainbow colours "—too much thinner has been used.
When the proper proportions are found, make note of them and use the same proportion for each mix.
Now get acquainted with your gun. Get the "feel" of it. Holding it near a practice surface, gently squeeze the trigger as you sweep your arm across the panel. For flat surfaces the gun should be adjusted to give a "fan" shaped spray. A. well-adjusted spray will have quite uniform distribution of material. However, the outer ends of the fan will have less material than the centre. If the fan is covering a 6-inch width, the outer 2 inches of each end will be fairly light.
The centre 2 inches will be quite "wet." In working the gun, keep all strokes parallel and overlap each stroke so that the material is of uniform thickness. Stop the flow of material at the end of each stroke or runs will result. That is the secret of successful spraying—the achievement comes with practice.
Three to five coats if using lacquer should be sprayed Onto the body, hood, and fenders, too, if they are to be the same colour. If a different colour is to be used on certain parts, masking will be required—but more on that subject, later.
The first coat should be very light. If too much material is sprayed on, it will tend to raise the oil undercoats. For that reason, have your material quite thin, and spray on but a mist the first time.
About 30 to 60 minutes should elapse between coats. However, if you are working alone on the car, your spraying can be practically without interruption. By starting, let us say, at the right front fender and working all the way around the car, it will take 30 to 60 minutes. Then you'll be ready to start the next coat at the right front fender.
Before going ahead with this, however, it is best to give the chassis, running gear, and under side of fenders two coats of chassis gloss black. The reason for doing this now is to guard against the harm that would result from the black blowing onto the finish of the upper part of the car. Of course, the spraying of the under part of the job may be done at any convenient time after having been washed and wiped off with gasoline. Also, if the engine is to be refinished, it can be given a coat of grey engine enamel when the chassis is being taken care of.
Holding the gun even a little too far away creates excessive finish dust and causes a rough surface. Holding the gun closer lets the finish strike the surface while too wet, thus tending to cause runs. Other things that cause runs is the use of too much thinner in the material or holding the gun in one spot.
And runs cannot be wiped out. Lacquer, unlike enamel, softens the under coats and becomes a part of them. That is a great advantage in the final finish, but a great disadvantage when runs occur, for it means that attempting to wipe the runs out may take off the finish down to the metal.
The only way to take out a. run is to let it dry thoroughly and then sand the spot smooth. This is done, of course, before the final coat is applied.
But until you are thoroughly experienced in mixing lacquer and enamel and in handling the gun, practice on junk pieces whenever you can. Some operators keep an old fender handy to use as a "target" to try out new mixtures—or different gun adjustments. It might be worth your while to do the same thing and thus avoid the chance of creating runs or sags in the finish of your work.
Masking for two-tone work
Any painter's supply store can furnish masking compound, masking tape, and paper. Newspapers can be used, but there is the danger of its printing on the surface of the car if much lacquer is sprayed against the outer side. The thinner has been known to soften the ink on the newspaper, thus making it print. Brown paper is not very expensive and is much more convenient than newspaper, being larger and stronger.
Masking compound is used on windows before doing any spraying. This compound can also be used to protect metal parts that are not to be sprayed. And some operators use it to protect the body itself in two-tone work. However, to get sharp edges it is necessary to finish off with tape.
After the parts that are not to be sprayed are covered, the surfaces that are to receive the other colour should be given two or three coats, assuming a lacquer job.
Then let the whole car stand for at least one hour, and then remove tape and paper. The masking compound will wash off with clear cold water. Use nothing but water—but use plenty of it. At this time the compound should be removed only from the lacquered surfaces of the body if this method of masking was used. Leave the compound on the windows and metal parts, for a final "flow" coat of lacquer thinner is yet to be applied.