As there have been many posts of late regarding painting/paints, I thought I'd put together some info covering the terms, techniques, etc. I hope some of you find it useful, and that it may help to clear up some of the confusion.
First, let's get some definitions established.
Types of paint:
Two broad categories: Lacquers and Enamels
Lacquers (Originally nitrocellulose based, now acrylic based): Were first used on cars beginning about 1924, and enamels came into use several years later.
Lacquers dry through evaporation (of their solvents), and therefore remain more susceptible to damage by solvents. In the 1950's when acrylic lacquers became available they became very popular as they were easy to shoot and repair and, to this day, some still consider it the paint of choice for custom work, though this is do more to myth/legend than any real advantage. Because as much as 85% of the lacquer shot goes into the atmosphere as VOCs (volatile organic compounds), they are now illegal in most states with air quality standards, and very inefficient in terms of material transfer (paint shot Vs paint actually applied to the car). Lacquers are very fragile. Even after months of drying, bird droppings, acid rain, and gasoline eat into the paint. Lacquer also dulls as it dries (as the solvents evaporate), and that's why you hear about classic Ferraris (and other classics) taking months to paint. Most pros doing custom work with lacquer will wait two or more months before buffing the paint to make sure all the solvents have evaporated. Even with the most meticulous care, lacquers can be very difficult to maintain. It's not for the daily driver.
Enamels: "One-Part" and "Two-Part"
Basically, enamels cure to some extent through evaporation, but they also cure by cross-linking of their molecules.
"One-Part" enamels (alkyd, or acrylic based):
Are fairly inexpensive, dry glossy so they don't require the hours of rubbing/buffing that lacquers do, but some (particularly the "bargain" brands) don't last long, and can loose their gloss after only a few months of exposure to the sun.
Non-Clear coated "Two-"Part" (catalyzed) enamels (epoxy or polyurethane, or
acrylic urethane, and others):
Though technically considered enamels, could be considered in their own category due to their special characteristics. These are paints like IMRON, and are polyurethane enamels. They set-up (cure) fast, are very resistant to solvent damage. The down side. Very difficult to touch-up/repair (especially if metallic and/or pearl), very expensive, very toxic and, because they dry very fast, if you're not careful (and quick), you can find yourself with a gun full of hardened paint, which can get expensive at $250.00 to $500.00 a gun.
Base Coat/Clear Coats: Not to be confused with "Two-Part" paints:
These are "Two-(or more)Step" paints, which can be either "One-part", or "Two-Part" paints. Remember, lacquers dry through evaporation (be they one or two _step_) and enamels dry through evaporation _and_ chemical cross-link. So, what's that mean Howard?? It means that a lacquer can be a "Two-(or more) Step" (base, metallics and/or pearls, and clear) paint, but it is _always_ a "One-Part" (no catalyst) paint. While enamels can be a "One-Step" (no clear coat), or a "Two-Step" (or more, ie. color, metallic and/or pearl coat and clear coat), and _either_ a "One-Part" (no catalyst), or "Two-Part"(catalyzed) paint. Most good (and most common) enamels are "Two-Part"(catalyzed). A good example of a non-catalyzed enamel would be model paint. It cures through _some_ evaporation, but primarily through chemical cross-link, though at a very slow rate when compared to a catalyzed enamel.
Base coat/clear coat paint systems are hard to beat for resistance to UV/solvent/chip/"bruise" damage, ease of maintenance/repair, DOI (distinctness/depth of image/"wetness"), and variety of "effects" available. Runs and/or sags in the clear coat can be eliminated with a "run razor", "nib file" and/or wet sanding/polishing, provided they're not too massive, and that you've put on a couple of "sacrificial" coats for polishing. Typically, when shooting clear coats, 1-2 coats are applied, and that's generally sufficient to achieve a very nice finish, provided you (or the guy you hire) knows how to shoot. However, if you want the absolute "t*ts", then lay down 2-3 extra coats to polish off via wet sanding (2000 grit or finer) and polishing.
Orange peel: Facts and myths.
This term has been the subject of many posts and much misinformation.
Facts: Causes are.
1) Improper gun adjustment and or techniques (too little air pressure, wide fan pattern or too great a distance from gun to surface)
2) Too high a shop temperature causing the paint to hit the surface dry inhibiting flow-out.
3) Gun "fanning" Basically too much air pressure and/or too little paint resulting in causing the
4) Improper flash time between coats. If the first coats are allowed to become too dry, the solvents in subsequent coats are absorbed into the prior coat before the paint can flow-out.
5) Incorrect thinner/reducer/basemaker: Under-diluted paint, or to fast a thinner/reducer allows the paint to dry before hitting the surface and doesn't allow the paint to flow-out.
6) Too little thinner/reducer/basemaker.
7) Improper mixing: If the materials are not thoroughly and uniformly mixed, the paint will not coalesce properly.
Myths: Orange peel is "good".
Total BS, unless you're trying to duplicate a factory/original finish. I suppose if your high dollar Ferrari (or other "exotic"/"collectable") was delivered with orange peel in the paint, and it means an extra $5,000.00 when you sell it, or those almighty concours points are what you're after, then by all means, have your painter reproduce the poor quality. But, if you want an outstanding paint job, then don't accept orange peel.
What to do if you're painting and you get some orange peel:
If it's in the base coat, correct it _before_ going any further. If it's not too severe, and you're not shooting a metallic or pearl, compounding may eliminate it. If it's bad, or you're using a metallic or pearl, you may have to sand down to smooth the surface, and re-coat. Make sure you've solved the problem (using a test panel) before you re-coat though, or you'll most likely have the same thing happen.
If it's in the clear coat and you've laid down enough paint to polish off a few coats for that show finish, then you should be O.K. But if you see it happening, stop and correct the problem before making things worse.
To strip, or not to strip:
Can you get a "good" paint job without stripping the car to primer and/or bare metal? Or, removing the trim? Or paying a lot of money? Or buying "expensive" paint?
If by "good", you mean shiny, then yes. But, there are other _important_ things to consider. How long will it remain shiny? How long will the paint stay on the car, how many chips (around the window gaskets etc) can you live with?
As far as painting over old paint is concerned, the key issue is total paint thickness after the new paint is applied, knowing exactly what's under the paint in terms of old paint/body work, and the proper selection/use of sealers/primers, etc. The total thickness of most paint jobs today is around 3.5-5.0 mills (0.0035-0.005") including primers, colors, and clears. If you get much over 7.5-9.5 mills (0.0075-0.0095"), then the paint is likely to crack, chip and/or peel. Again, unless you know what's under the paint in terms of quality of previous paint/body work/materials, you're taking a big chance with the money you're spending.
If you want to know why a good paint job can be so expensive, go down to the boneyard and get a fender off of car. Then, remove all the trim, strip the paint, do any necessary bodywork, and repaint it. Be sure to buy the good stuff (about $250.00+ per gallon for just the clearcoat). Then, once you're satisfied with the result (make sure to be as critical/picky of yourself as you would be of anyone you might pay for the work, about every spec of dust etc.) add up (honestly) what you're time/money is worth. Don't forget to take into account the time/money it took to learn what you're doing (your salary is based, at least in part, on your education, isn't it?) the cost of the tools/equipment, utilities, rent, insurance, salaries etc. for employees, all that stuff. Now, multiply just the time/effort in labor for only the work itself by 10, at least. Also remember that geographic location can play a part in costs (just as it does in real estate, etc) due to state/local regulations. That should give you an idea of why paint jobs can cost what they do. Just because a car is only worth a few thousand dollars, it doesn't make it any easier/cheaper to paint, and that's where I think much of the "problem" lies in regard to painting 928s. It's a bit hard to justify $5,000.00 for a paint job on a $10,000.00 car. It has to come down to this. Do you want a "Macco" paint job, or a "Porsche" paint job? If you choose cheap materials/methods/"shortcuts", you can't really blame the guy doing the work for less than perfect results, unless of course it's you that's doing it. For what it's worth, in Germany, painters are taught to always strip off the old paint before doing a repaint.
Hope this has been of some interest/help.
Dave's 928 Custom MetalWorks