Paint repelling?

[cruizer]

The other day I was doing a bath of parts in Black Magic.
I was noticing on some of he items, where the hook was connected to the part, the paint was actually repelling off! There was a bare spot, and I shot paint right at it, and you could see it just turn away from it! Like opposing magnets. after while, this stopped and I was able to paint the part.

Has this happened to anyone else?
Anyone know what causes this?

[Blademan]

Re: Paint repelling?

Theres a tech term for it, dont remember what it is but basically a part can become charged enuff that it will repel the charged particals...usually happens in tight recess's..

[c130herc]

Re: Paint repelling?

When I coated all my kitchen cabinet hardware it would happen to me in about 1 out of 20 pieces. Just figured the earth?s magnetic field shifted slightly.

[Popeye]

Re: Paint repelling?

"Problems with powder charging or delivery will make it more expensive
and difficult to apply the material to the part. Many times these
problems can be corrected by a review of the basic function of the
application system to see what is wrong. This troubleshooting section
covers some of the common mistakes that cause difficulty in the application
of powder coatings.

Poor Charging
When it becomes unusually difficult to get powder into the inside corners
(Faraday cage areas) there are three common things that should
be checked.
1. Grounding
2. Powder flow rate
3. Voltage
Grounding may be lost from excess build-up of coating on the hangers.
Measure the resistance from the part to the conveyor rail with an
ohmmeter. If the resistance is greater than 1 megohm, the path to
earth ground is not adequate. Check the components of the racking
arrangement, find the insulated component and clean it to make good
contact.

Excessive powder flow rates will cause a reduction in charging efficiency.
Check the gauges and reduce the flow rate.
The voltage at the gun tip needs to create a field of high potential.
Check voltage at the gun tip to make sure that it is compatible with
the gauge.

The Faraday cage effect describes the problem with getting the powder
to adhere to the surface of an inside corner. This is the result of
resistance created by the presence of an electrostatic force. The elec-
X

Application Problems
Etrostatic force is attracted to the prominent areas of the part where
the resistance is low. Free ions and powder film build quickly on these
more prominent surfaces and create additional resistance to penetration.
Faraday cage effect can be reduced by limiting the current draw of the
gun, by using the correct nozzle for the task, and by control of the
flow rates and gun-to-target distance.

Back ionization
Back ionization occurs when the electric field on the part surface
reaches a point where it begins to ionize the air under the powder
film. Positive ions are released, creating ruptures in the film, repelling
the powder and causing a blemish that is commonly called a ?star.?
The potential causes are:
1. excessive voltage (high current draw)
2. the gun is too close to the part (high current draw)
3. too much film build (too many free ions on the surface)
4. application over an already coated surface (dielectric of coating
creates resistance)
5. poor grounding of part (no path to ground for ions)
Preventing back ionization requires control of current draw. If the current
draw is too high the part surface will not be able to take the
number of ions arriving. Voltage levels, gun-to-target distance, the
presence of ion collectors or the control of the load line inside the gun
affects control of current draw. Lower voltage, greater distance to the
target, automatic current control or ion collectors can be used to reduce
the potential for back ionization.