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Old 04-02-2009, 06:00 PM
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Grit - how fine is too fine for adhesion?

I believe the general consensus here is that sanding your primer to 400 grit before painting is OK and that 600 grit is highly preferred. Some have even said they go to 800.

I am currently sanding my sedan/delivery's primer to 600 grit. HOWEVER, I've noticed that as the sandpaper wears out or gets somewhat clogged, it SEEMS to sand finer and finer. New paper - duller, scratchier looking finish. Old paper - smoother, shinier looking finish. I'm somewhat of a tightwad, so I try to get every last ounce of use out of my paper. The result is that I can tend to create some very shiny surface areas.

So my question is, am I really sanding beyond 600 grit when I push the paper beyond it's normal life span or is it just my imagination that I'm creating a finer grit. And more importantly, can it become detrimental to adhesion? How fine of a grit can you safely use without getting into problems?

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Old 04-02-2009, 06:39 PM
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Personally I went no higher than 600 on the primer of my 34. When I noticed the paper not cutting very well I discarded it for a new sheet. You are spending big bucks on paint, don't cheap out on sand paper at this stage.

When the guys I paid to wetsand my clear started they used 1000 initially, but with a new sheet they rubbed it back and forth onto itself before hitting the car with it. Did the same with 2000, then hit it with Mirka Abralon 4000 pads wet. Even before hitting it with a buffer it had a sheen to it.

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Old 04-02-2009, 08:19 PM
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I usually sand to 400 grit and then hit it with white scotch brite pads they have 3 types fine, med, course. never had a problem, I also use a mild DuPont wash & wipe then a sticky type tack rag before shooting paint. cant wait to see some color cboy
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Old 04-02-2009, 08:58 PM
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Um, just so this thead stays on track...my questions are:

A) Does worn paper sand at a finer grit that when it is new and can that potentially result in adherance problems.

B) How fine a grit can you use before it will hinder adherance of the base coat.

We have pleanty of threads in the archives about the best grit to finish up with. So that isn't my question. I'd like to keep this on just A and/or B above.
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Old 04-03-2009, 05:00 AM
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It's not an adhesion issue.
You can get enough adhesion painting over paint only scuffed with a
fine gray Scotchpad., if it's really clean.
It's done all the time when blending new paint into existing.
A lot of times those areas aren't even sanded by shops and they don't
have adhesion issues. Paint will stick to paint-if it's clean.
The advantage of coarser grits is the leveling, or straightening.
You can't get it as straight with finer grits, like above 600.
even with a hard block it's difficult.
But with coarser grits, it's much more likely to be straight and it's
easier to sand.
Sanding primer, you want to flatten it to be straight, not just smooth.
Coarser grits probably do increase adhesion, but that's not an issue.
either way the paint will still stick and hold just fine as long as it's clean,
that's the bigger factor, being really clean.
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Old 04-03-2009, 05:40 AM
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If you were to look at the sanded surface with a decent magnifier you would see that the worn paper is not leaving a crisp anchor pattern, it will be shallower and have a less ragged edged.

A fresh finer grit will leave a smaller anchor pattern but it will still be crisp and ragged, just not as deep and a lot more of it.

As the grit gets finer the anchor pattern gets shallower and a result of that is with the smaller valleys the surface is more reflective.

As the grit gets duller the valleys also get shallower and will not have the sharp edges, both of which make the surface more reflective.

Those valleys are farther apart than the valleys of a surface sanded with a sharp finer grit although the reflectivity will appear similiar.

The surface of the paint will look more like rolling hills and valleys instead of a small peaks and valleys.

The end result will be a shiney surface either way, although one will look wavey the other will look like glass to the naked eye.

Holding power, I have no idea, as I have never seen the results of a scientific test to be able to answer your original question.
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Old 04-03-2009, 07:31 AM
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Quote:
Originally Posted by jcclark
You can't get it as straight with finer grits, like above 600.
even with a hard block it's difficult.
Very helpful info JC and Oldfool. One short followup question. As you progress from coarse to finer grits, can you actually start to undo your straightness if you get above 600 (or use worn 600 paper)? Or is it just more difficult to get straight if you START with a grit above 600?

In my case, for example, I've basically gone 120 - 240 - 320 - 400 - 600. I used 3M dry guidecoat at each level EXCEPT when I got to 600. I found at 600 I was pretty much able to immediately see any flaws or low spots in the 600's scratching pattern and that the guidecoat did not seem that helpful at that grit.

Anyhow, I'm pretty confident, based on my use of dry guidecoat, that my 400 sanding had things straight and that my 600 sanding with NEWER paper also maintained or improved that straightness. I'm just curious if by pushing my paper too far (using it worn) or by going to a finer grit and getting a much slicker looking surface, I could actually start doing more damage than good.

I like the shinier surface because you can stand back, eyeball the panels and get a pretty good idea how straight things are going to look once they are painted. But can those looks be deceiving? Or put another way, is a person better off sticking with coarser grits/newer paper and trusting only the guide coat method as your guide or are you better off creating a slicker looking surface with finer grits (or worn grits) and being able to observe the panel with your naked eye?

(Perhaps I'm over analyzing all this...but I'm very curious about how to achieve the best results possible. Oh, and I also live in constant fear I've messed up a perfectly good job with some boneheaded move.)
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Old 04-03-2009, 07:58 AM
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I think you have followed good practice by using courser grits first then working your way up to 600. The coarser grits have done the job of leveling if used with sanding blocks and guide coat. Now you have to worry about sanding scratches left by 320 or 400 that the paint will shrink into and be visible. That is where the 600 comes into play.

Vince
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Old 04-03-2009, 08:41 AM
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Quote:
Originally Posted by cboy

I am currently sanding my sedan/delivery's primer to 600 grit. HOWEVER, I've noticed that as the sandpaper wears out or gets somewhat clogged, it SEEMS to sand finer and finer. New paper - duller, scratchier looking finish. Old paper - smoother, shinier looking finish. I'm somewhat of a tightwad, so I try to get every last ounce of use out of my paper. The result is that I can tend to create some very shiny surface areas.

So my question is, am I really sanding beyond 600 grit when I push the paper beyond it's normal life span or is it just my imagination that I'm creating a finer grit. And more importantly, can it become detrimental to adhesion?
Dewey, I am no expert on paint prep; but will comment on what is occurring once the paper is "used up" and yields a smoother, shinier looking finish.

Since the grit is no longer 'cutting', you are now burnishing the surface. You could do the same thing by just using the back side of the paper where there is no grit.

It is not really your imagination that you seem to be creating a finer grit. There just isn't any grit left to do any cutting.

Since you want some surface for adhesion, you may be better off by changing the paper before it is worn out.
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Old 04-03-2009, 08:51 AM
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Dumb question; does anyone have problems w/ paint not filling 400 grit surface and leaving scratches showing thru? I have always final sanded primer w/ 400 then put on color and never have had print-thru scratch problems. If that's the case, why go to finer grits that don't level as well and seem to dull and plug more easily?
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Old 04-03-2009, 09:47 AM
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Quote:
Originally Posted by willys36@aol.com
Dumb question; does anyone have problems w/ paint not filling 400 grit surface and leaving scratches showing thru? I have always final sanded primer w/ 400 then put on color and never have had print-thru scratch problems. If that's the case, why go to finer grits that don't level as well and seem to dull and plug more easily?

I'm having the same problem. I had some dust after painting the cowl, wet sanded with 600, re-shot, a lot of fine scratches in the basecoat. Would the clear negate the scratches. I quit after the last results (yesterday).

I'm kind of in the same boat as Dewey right now, I am wanting to do my final sanding, I plan on using 600, wet. Yea or Nay?
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Old 04-03-2009, 09:55 AM
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This may be an exercise if futility since my camera skills don't allow me to show you the primer surface in microscopic detail. Also, I know it is always difficult to read a situation based on pictures...particularly lower resolution internet pictures...but I'm conflicted now between Vince's "followed good practice" and Frisco's comments on burnishing the surface and not using worn paper.

So I'm posting these pictures just to see if based on the reflectivity, folks think they look fairly typical for primer at 600 or if I've burnished the surface way too much and I might be better off going over everything one more time with all new 600 paper.

Vince is certainly right about one thing, I feel at this juncture like I've really gotten after most of those deep sanding scratches left by the coarser papers. Any flaws or scratches really start to stand out when you get a shiny surface like this and you can really find and eliminate them. But as Willys says, maybe I don't need to be removing those scratches in the first place.

Starting to make my brain freeze up.

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Old 04-03-2009, 10:04 AM
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Quote:
Originally Posted by willys36@aol.com
... I have always final sanded primer w/ 400 then put on color and never have had print-thru scratch problems.
Again, I don't want to be rude and stiffle a good discussion, but I also don't want this thread to start wandering into a major debate over "best last grit". There have been lots of excellent threads on that in the past and in this case 600 grit was selected. End of THAT story. The question here is wherether or not "burnishing" (sanding with worn paper or even finer grits than 600) can actually create problems that need to be addressed before shooting color.
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Old 04-03-2009, 10:34 AM
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A I throw the paper out when it don't cut or leave a consistent pattern
B I have no idea than personal experience
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Old 04-03-2009, 10:36 AM
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