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Contamination would include things like silicon and other repelling type of materials.   Waxy and greasy surfaces will repel paint and not allow it to grip to the surface under the wax and grease.  Any kind of repelling contamination as well as other forms of dirt need to be removed prior to painting.
Adhesion Testing - part 2

  • Wet surfaces:  Wet surfaces will repel oil paints and primers (oil and water don't mix and if put in a container will separate from each other) and neither latex or oil primers or paints will not be able to penetrate into a surface that is wet which will greatly reduce mechanical adhesion.

    When it comes to adhesion always remember the "clean , dull, dry" rule of surface preparation. 

     Now, lets look at the field test for checking adhesion so we can avoid problems before that can occur.

    The cross cut test.  There are two variations of the cross cut test, one is to simply cut an "x" into the paint film with a knife or razor and the other is more through and it calls for the cutting of a "tic tac toe" type of cross hatch grid into the paint film.  I believe the "official" cross cut test uses ten vertical and ten horizontal cuts   Both tests call then for a piece of tape (there is a ASTM standard for the tape but I use  masking tape which has good tack or adhesiveness- not the blue easy release tape , or duct tape. Use good tape with strong tack for this test or you will only be fooling yourself.  The way the test is performed is after the cuts are made, the tape is rubbed down onto the paint and then is abruptly pulled off with a hard pull.  The paint should not pull off if the adhesion is good.  The cuts provide the tape the ability to lift off the coating and thus the multiple cut cross hatch which has more cuts is a better test than the quick "x" cut test.  I usually will rub down and pull  off a few times per spot just to be sure I've done a good test and have real results.  If after doing multiple pulls in multiple sites on the surface passes - then I know I can proceed with the rest of the priming and then paint.  If it fails then I have to continue to clean further, which will then be followed by another round of adhesion tests., and so on.......


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A word about adhesion testing:  all paints need time to cure to develop full adhesion.  Latex paints, although they dry quickly- cure very slowly and will not pass a tape test for at least two weeks, maybe three or four weeks or longer.  Oil based paints will usually give a true result after a few days to a week.  If the paint fails a tape test you may only need to wait a bit longer for the paint to cure more and then it will pass the test.  So try it again in another week and if it passes then you are all set. If it fails again you may have to wait a bit longer and try again or maybe you have inadequate adhesion.
The darker the paint the more glycol from tinting colorants there will be in the paint and the longer the paint will need to cure before it will pass a tape test.
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