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Old 04-10-2009, 10:29 AM
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Testing sanding grit for adhesion.

Over in this thread we had a discussion of the potential drawbacks of sanding primer TOO smooth by using worn paper. Bee4Be suggested doing an adhesion test (applying duct tape and/or masking tape to see if the paint would lift) to determine if the surface being painted was "too smooth".

Well, this morning, as I looked at my messy paint gun, a "Mythbusters" test occurred to me. The chrome on the handle and the polished aluminum on the cup are very very smooth. Much smoother, I surmise, than any 600 grit sanding job...even with worn paper.

So I thought hey, this will be a great way to run a little adhesion test and clean up my paint covered gun all at the same time. Just put masking paper on the gun and when I pull it off I'll have a sparkling new tool. Paint can't POSSIBLY adhere to a surface as smooth as chrome.

So here are the results of my little experiment. Photo 1 is the "Before" shot. Photo 2 is with masking tape (3M #2015) applied. And Photo 3 is the result. Although I didn't photograph them, I also did the identical experiment using 3M 2080 Painting Tape and again with two brands of duct tape. All experiments had the exact same result. Nothing. Not one particle of paint lost adhesion to the super smooth chrome finish. My gun remains butt ugly.

I submit this test only partially tongue in cheek. I've already re-scuffed the sections of my sedan/delivery to 600 grit that had been burnished much smoother and I'm not advocating that anyone do what I did. However, if paint and primer can hold up to the adhesion test on a super smooth finish like chrome, I find it difficult to believe it wouldn't adhere to 800 or even 1000 grit sanding.

Always learning...and sharing what I've learned.
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Old 04-10-2009, 11:07 AM
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People get to hung up on grit size with regards to adhesion.
Yes it can make a difference , but cleanliness is the biggest factor.
I've seen clearcoat adhere perfectly fine to unsanded,
unscuffed, shiney clearcoat. But it has to be clean.
It's hard to get something clean enough without scuffing
or sanding, but it can be done.

I now do all my clearcoat blend transitions on the old paint that has only
been buffed, not sanded at all.
I don't have to buff that area after blending that way.
I think it actually holds up better, longer then if it were sanded..
The old paint is buffed to make it clean,
buffing is just a finer grit of sanding and clear sticks to it just fine.
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Old 04-10-2009, 11:15 AM
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In my field we use this test alot to determine acceptability of the paint surface when it is new and old, but we first use a utility knife to make cuts to the surface in a 90 degree pattern with the cuts being about 1/4 or 1/8 inch apart.
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Old 04-10-2009, 02:04 PM
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