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Originally Posted by M540i LE
OK Wout. Why? Genuine question, as I am looking at repairing a (slightly) poor paint repair on the rear wing of my 540i and am concerned about colour matching etc.
Interested in your thoughts.
Cheers
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Dave,
Its not very hard, but forgive me for being a bit theoretical about this( you guys are not used to that from me!) but FAPPOV ( from a painters point of view) its like this:
The color that is on your car is metallic. Each metallic paint , whether it being Acrylic,latex or water based,
needs lacquer to function, as one can barely blend hardener into the color. The laqcuer bonds with the color ( 2 component , 2K) in order to make it dry. Non-metallic colors ALWAYS have hardener blended into the color itself, often labelled 1K ( incorrect, but thats another story). No lacquer, no REAL dry 2K paint
Next thing is, virtually all metallic paints
shine through. This has something to do with the structure of the indiv. components that must allow eachother to be visible when blend. Neg side-effect of this is, when respraying a certain part of a car,the surface one applies the paint to affects the end result. Therefore, simply adding the the "same" color on a wing that is already sprayed in that color will cause color differences. It's the same as putting transparant plastic bags over one another, in the end you will end up with a package that is not transparant anymore, each layer contributing to the effect. This very phenonenom is the reason why we even use 56 different color coded primers these days, as you want as little material on the surface to prevent structural differences. Fitting brand new wings to a car that is sprayed already is so hard because they come in black. One must anticipate the color blend ( there are several shades available "unofficially" for most colors, Kawasaki green has 28) because of the primer layer. 8 out of 10 times, this turns out to be very difficult , there will always be at least one angle that show shade difference.
^ the paint shop that did this car choose for the rear angle view to be out of shade, a common choice when costs need to be saved ( company cars FI)
Finally, silver is by far the worst color to spray. BMW's usual suspects are Sterling Silver, Titan Silver and Arctic silver. Sterling is 2nd worse next to Titan. This problem is caused by individual
metallic components to "fall" to one side or another.This is also caused by the material becoming electrostatically charged in the sanding process. BMW uses polygonal shaped metallic particles, whereas lots of other paint manuf use either hexagonal or rounded crystals. It may seem that polygonal prevents the problem described, but the multitude of reflections makes it hard. Also, because one must try out the layer thickness in order to get the right shade, causing the lower layers to cast a shadow on the top ones. This problem particularly occurs when vapourizing too soon after applying a full coat, in fact the thick layer builds up a spectrum for the vapour to shade. It is virtually
impossible to achieve the same effect of metallic component positioning within the paint compared to surrounding surfaces. This is the main reason for "vapourizing" into other panels, in order to "blur" the wrong ( 0.001% wrong that is!!) paint into the original paint. Other nice side-effect of silver is that it actually changes shade from wet untill dry 9 out of 10 times.
(Note: 8 out of 10 people won't even notice the paint being slightly off untill 10%!!)
In practice, it means that one must :
1) ALWAYS try to avoid silver paint ( or other light transparant metallics) to be sprayed right untill the edge of a panel. If this is enevitable, same rule applies to the next surrounding panels. Chances can be taken however when it comes to for instance the "top" of the wing and the bonnet, as the shade differences will mostly show on vertically placed body parts. You don't have to vapourize on the bootlid when spraying the quartel panel with 99% right silver color. IMO you should apply lacquer to the bootlid when doubting the color. This will mellow the effect when the color turns out 95% inst of 99%, because of the surface structure matching.
2) NEVER apply the same amount of lacquer on the "new" paint incl the vapour section and the original paint. For instance, spray the wing 25% primer, 75% silver, 75% thick lacquer and 25% ( the last part without new paint or primer) on the OEM paint. When one ignores this rule, the transparant plastic bag effect occurs again, causing the lacquer to change the shade of the paint.
Visually, painting a wing for stonechip trouble looks like this ideally:
However, if the wing is damaged so bad that it needs a full coat of respray, the following picture would be ideal :
^ in this picture, it is stated how rough the surface must be sanded. Primer must be sanded 260 grain up, in order to make the stonechips disappear.From the primered part up untill halfway the RF door must be sanded with no less than 400 up to 550, just to make "room" for the paint. Don't take the rate too high, as color coat will only stick up to 600 grain, any lighter and it won;t stick on. The last part of the door however, will only see a VERY thin layer of lacquer, because the OEM lacquer will remain on there in contrary to the wing and the first part of the door. When applying too much lacquer there, the shade difference between the RR and RF door will show, causing you to shift even further!! Sand it with 1500 or higher, as a very thin clear coat will stick more easy than color coat together with clear on top.
3) ALWAYS disassemble all trim parts and door handles on the parts in question.. Masking them is possible, but will prevent you from sanding underneath causing lacquer trouble in the future AND leaving a possible dustpit for when the spray gun blasts onto it. Also , lacquer has the tendancy to built up when it flows onto a masked area, leaving small ridges on the trim surround.
In the past, ALL things described above have gone wrong with my car :cry:
Bear in mind that this is probably over the top when it comes to quality for 75% of the people in the world. Just decide which group you belong too!
cheers,
Wout
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