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).
Last edited by BarryK; 04-07-2005 at 12:41 PM.
Reason: Phone call