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Old 10-02-2006, 05:26 PM
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spraying over sanded OEM finishes ( no primer )

When i bought my paint, the guy said i didnt have to prime the whole car, he said i just had to sand it down. So thats what im doing, BUT, what happens when i poke through the clear and into the base coat/primer? Is this going to manifest itself in the final finish, and if so, how? What about places where i sand through to bare plastic (bumper etc). Im pointedly attempting to NOT sand that deep, but it happens from time to time. Will this cause surface discoloration or will it cause the paint to flake off?



Also, a random question.
Ive parked my car under the L train track in chicago on several occasions. Whenever a train passes overhead it causes a "mist" of rust colored dust and tar-type stuff to shower down upon unsuspecting cars. Many a time have i returned to my car to find it a slightly browner color. Although the dust is quite tenacious, it does come off with careful cleaning. My question concerns the tar. It seems that on one spot on my car, the tar has "sunk" into the paint. I scraped off the majority of it, yet it seems to have been absorbed by the base coat. I have sanded the area with 400 grit paper, past the clear coat and into the base coat yet i can still see a slightly yellowish discoloration. If i paint base over this will it be adversly affected?




The paint I chose is PPG Omni AU, BC/CC system


any advice is greatly appreciated

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Old 10-02-2006, 06:11 PM
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You shouldn't have a problem painting over the factory finish. I haven't noticed a problem when I've sanded down away anyways. Most of it you probably won't be going through the clear or will be spot priming those areas. You should primer or epoxy any bare urethane or plastic spots you have. It could soak in there leaving a noticeable spot when painted. That being said, If you are changing color and will have quite a few primer spots, I would still seal for a few reasons, To lessen the chance some spot could give you problems, to get the car all one color close to what you are painting not having a different colored primer spots then base. Omni is not known to cover well, and I think you are going to want it all one color. If a medium dark grey is kinda close in shade to the color you will be spraying, the omni epoxy doesn't seem to bad and could be reduced as a sealer. I think the little bit invested in a sealer would be a good idea. I haven't bought omni epoxy in a little while but think it is around $25/qt and 2 quarts should be enough for the outside of most cars. Paint is more expensive then primer so you will be further ahead sealing then blowing a lot of paint on to get coverage, besides lessening any possible risks you would have. I'd check on the rail dust. Its been many years since I was in the dealer and ran into, but we a special cleaner to get rid of it, but those werent repainted. I don't remember doing anything special on any cars that were repainted. Not sure if it would cause any sort of problems if with painting, think cleaning and sanding would get rid of it for a repaint, but not positive on that.
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Old 10-06-2006, 07:23 PM
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The rail-dust damage I've exsperienced was like a burn. It was like you stuck a hot punch on the surfase of the paint and it burned down into the color. I've just sanded them down, sealed them with epoxy, and painted them. Never had one come back.
Good Luck!
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Old 10-06-2006, 08:18 PM
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The spots that "CMC" mentions, may in fact be burns, if they came from parking under rail tracks. The train cars, when the run down the tracks are grinding the wheels against the tracks. It's like grinding metal, and having sparks fly against a car parked nearby. Although, I have seen metal embedded into the paint from grinding, if the pieces aren't hot enough, they often don't stay after they make their marks. They simply blow off or are rinsed off with water.

When I lived in Chicago, I never could convince myself to park under the tracks. Always figured, with my luck, something would fall off the train just when it went over my car.

As long as there isn't anything embedded in it, that sticks out, I don't see any reason to not just seal it with epoxy and go on.

Aaron
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Old 10-07-2006, 09:38 AM
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Quote:
Originally Posted by Saddlebacks
When i bought my paint, the guy said i didnt have to prime the whole car, he said i just had to sand it down. So thats what im doing, BUT, what happens when i poke through the clear and into the base coat/primer? Is this going to manifest itself in the final finish, and if so, how?
It may or may not leave a ring, depending on a lot of factors. Primering these spots is the best practice. If primering sand-throughs will not be done due to time constraints, it's good practice to use a sealer, preferably one that is as close as possible to the color or your topcoat.

Quote:
What about places where i sand through to bare plastic (bumper etc). Im pointedly attempting to NOT sand that deep, but it happens from time to time. Will this cause surface discoloration or will it cause the paint to flake off?
Depending on what kind of plastic you are dealing with, there may be loss of adhesion in those spots. An adhesion promoter like Bulldog would be best. Again, primering these spots after applying the adhesion promoter is best practice, if not, then sealer. Painting directly over sanded featheredges is putting a lot of faith in the quality of the featheredging and the ability of the paint to cover it up without showing anything.
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Old 10-07-2006, 10:12 AM
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I am wondering if the rail dust would be removed off the surface of paint with a clay bar, I would think so.

Brian
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Old 10-07-2006, 09:50 PM
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i decided to epoxy the whole car, thanks for all the advice
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