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Old 04-07-2005, 11:31 AM
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Adhesion test.

Adhesion test:

A month ago someone asked for an adhesion test to decide how well clear sticks to factory clear sanded with different grits of sandpaper.

Here is what we did, to be fair we found a factory painted truck side off a 2003 dodge the color was silver although that is not important, what is important is this panel was fully cured so instead of painting over fresh paint in witch case you would have better adhesion.

We cleaned the panel 4 times as an over kill before we sanded and that was after we buffed the panel with a cutting compound to remove any weak or dead paint that we could not see.
We taped off in one foot wide sections and wet sand the sections with starting left to right with 320, 600, 800, 1000, gray scuff pad, 1500 and 2000.

We than sprayed two wet coats of a HS polyurethane clear with a mill build of 3.8-4.0 average across the panel. Panel was kept at 75 degrees for 7 days.

Best way to check adhesion is with the stone chip test, measured weights of stone with 4 different sizes dropped from 8 feet. This is a radical test as new base clear with some companies will look like you shot the panel with buckshot and some paints won’t chip at all.
Keep in mind, polyurethane has much more adhesion and flexibility than acrylic urethane but across the test it should be consistent.

The results we had ONE chip in the 320 and SIX chips in the 2000 and the rest had no chips but some film scaring.
So a guess would be the best range of sand paper is 400-1500 for maximum adhesion.

To finish up before I was interrupted the test says to me the 320 is the beginning of weakness as you go courser, I wanted to test 400 but did not have room but as it turned out, my guess is the 400 would not have chipped.
The 2000 is interesting as it got crazy going from 1500 to 2000 and I used 3M 2000 paper only because its probably the choice of 80% of the body shops so thats why I went with it although I never use it myself.
Don't let the six chips in the 2000 scare you as that is still better than some base clear systems after seven days and I'm talking premium base systems (foreign).

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Last edited by BarryK; 04-07-2005 at 12:41 PM. Reason: Phone call
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Old 04-07-2005, 04:17 PM
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Interesting info Barry, that's why I keep coming back here. Thanks for sharing the test results. I've often wondered how the clear adhesion was affected by different surface textures. I would think there must be an ideal grit for each product, 800 grit would be in the middle of your range. I've used 600 and 800 for final prep for years.
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Old 04-07-2005, 09:03 PM
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Great threat Barry, thank you for sharing.
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Old 04-07-2005, 09:34 PM
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Good stuff. Do you have or do you know many people who use some sort of color blender, example DBC/DBU 500 or other brands equal? What do you think this would do for adhesion? I only ask because the question asked was about factory sanded clear as would be needed for a blend on a repair whether it be blend within panel or blend panel.
Like you said fresh paint would have better adhesion, so would the grit matter as much on a flow coat type situtation? I like to use 600 grit on a flow coat but have done the same over 2000 grit. As far as adhesion is there a difference in this case?
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Old 04-08-2005, 04:10 AM
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Not sure about the color blender but a lot of people do ues it?
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Old 04-08-2005, 05:31 AM
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The DBU500 is more of an intercoat clear than a blender but it can be mixed with DBU color to lessen the strength for blending. DBC500 is very similar, and Global D895 is also similar. Most people when blending problem metalics will apply a coat of this transparent base over the complete panel then blend the color into it. This avoids metalic hangup in the sandscratches. Since these three products are basically just transparent basecoat I'm sure the adhesion with them would be the same as if you were just coloring the complete panel. Dupont's 222S has also been used for the same function since the days of laquer. The DBU500, DBC500, and D895 can also be used for checking color match when a quick coat of clear is needed.
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Old 04-08-2005, 07:01 AM
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Thanks Barry.
A question.
What if any difference did you see in the "appearance" of the clear across the panel in reguards to the different grit scratch's?

I was wondering about the overall effect the different depth's had on the actual finish/'peel/clairity.
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Old 04-08-2005, 07:30 AM
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Was not really looking for that but no scractes hit me in face.
All looked same except cleared over the dividing 1/4 fineline tape.
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Old 04-08-2005, 10:26 AM
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This is interesting and now I question which is better.
I have always wet sanded the entire panels existing clearcoat with
600 before making a spot repair and reclearing the whole panel.
It was suggested to me on this forum to use a gray scotchbrite
pad instead of the 600 wet sanding for existing panel prep.
Is there any reason I should stop sanding with the 600? after
seeing these test results it sounds like it would be better than
the gray pad.
I know the wet sanding is slower but it does level out the surface better
and seems to be more "scratches per inch" (better adhesion?).
This is strictly a new clear over existing factory clear question.
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Old 04-08-2005, 11:24 AM
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I think this test just kinda confirms that it does not really matter what you use over factory clear as long as its somewhere between 320 and 2000.
Further it confirms That Grey pad most shops have been using for years, works perfectly.
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Old 04-08-2005, 11:30 AM
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Thanks!
I kinda thought the same thing.
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Old 04-08-2005, 04:01 PM
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Thanks Barry, these tests are really great to understand what IS better.

In the old lacquer days a "color blender" was a very slow thinner with some clear in it for "melting" the edge. These days "color blender" (it seldom is called that, but you get the idea) is many times only a "colorless basecoat". DuPont had a recommendation for years with Chromabase to reduce the mixing clear off the color bank with basemaker 1:1 just like a color and apply a coat over the panel prior to blend. It fills the sand scratches with a clear base coat then when you apply the base and blend the metallic doesn't "stack" in the scratches and cause a halo looking blend. Now there are many who sell a RTS or similar product that is basically the same thing.

Brian

Last edited by MARTINSR; 04-09-2005 at 09:49 AM.
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