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  #91 (permalink)  
Old 06-13-2013, 05:38 PM
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I guess Brian this is what I'm getting at...I maintain that by sanding a surface, you are in fact opening it up and allowing the chemicals to enter the surface and giving you your chemical adhesion...The sanding gives you the mechanical or Physical adhesion.

I did a test many years ago...I took a panel that had been cleared, 1/3 I left alone..the center 1/3 I sanded with 400 grit, the other 1/3 I sanded with 2000 grit...I then applied 2 coats of clear over the panel.

The part of the panel that hadn't been sanded, the clear came off with a razor blade...The areas of the panel that had been sanded with 400 grit and 2000 grit it was difficult to tell which area had better adhesion, why, because the panel was opened up by sanding and both types of adhesion where present.

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  #92 (permalink)  
Old 06-13-2013, 05:45 PM
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So I look at that experiment and think

--I don't know how it proves chemical or physical adhesion (meaning, it could only be one, or the other, or both, but there's no way of telling from that experiment). You would have to control for one condition.

--I would think a good test would be sand part of something, leave the other part alone. Go over with a solvent. If it softens up, it would be prone to chemical adhesion as the solids are being dissolved. If it doesn't soften, I would not think it would be prone to chemical adhesion

--I do not understand how the top crust would be different than the center of the substrate. It seems it should be a uniform layer of solids, with the same porosity through and through.
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  #93 (permalink)  
Old 06-13-2013, 05:56 PM
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As I'm posting, I'm also sanding one of the panels that I used for testing the SPI Epoxy..it has base coat and clear on the panel as well...I will sand 1/2 of the panel and leave 1/2 shiny, the panel is just over 6 months old...do you think I should pour reducer on the panel or let it soak in reducer for say an hour/

Ray
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  #94 (permalink)  
Old 06-13-2013, 07:46 PM
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Originally Posted by Lizer View Post
No I'm saying that what makes sense to me is there is only physical bond when a cured finish is sanded. I'm also saying that I don't know if there actually is any chemical bond at any point, unless more coats are added to a finish not completely cured so it is still susceptible to (dissolved by) the solvents in the newly added coats.

The chemistry and science of paint finishes fascinates me, and when I work on my car, I really try to apply the science I know with the art and skills I try to learn here and elsewhere. I just wish I knew more about the actual chemistry. What I'm saying in the above paragraph are just musings...educated guesses maybe, but don't accept them as the truth. I'm not a chemist.

What else could it be if the solvents from the next application, be it paint or clear or what ever, what could it be if not a "chemical bond"? If the solvents are going into the substrate "joining" with the substrate, what else would it be called? Is there another "level" of this that WOULD be "chemical bond" but simply solvents soaking into the substrate isn't?

Brian
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  #95 (permalink)  
Old 06-13-2013, 08:23 PM
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Brian, this is a real point of interest for me. When I've gone to these training courses I always asked many questions. This topic was brought up many times and the way it was explained to me was that by sanding, your opening up the pores in the paint and when you apply more paint the solvents in the freshly applied paint permeate the sanded substrate bringing with them the active ingredients that form the chemical bond. I have taken the test panel and soaked it in reducer for 15 minutes. The sanded side is sticky and starting to lift on the edges...I then took a razor blade and around the edges and with a bit of effort I can remove the clear and the base. The Epoxy doesn't seem to be effected. The side that wasn't sanded, the edges are fine, the clear seemed a little stickier than before but a razor blade wouldn't touch it unless I gouged the clear. 10 minutes later, the clear wasn't sticky at all.

Now, I have been told the same story by different manufacturer's as to the benefit of sanding. They have all claimed that it isn't the size of the tooth that gives you the mechanical adhesion...it's more the number of teeth...that's why you get mechanical adhesion even with a fine grit of paper (a finer sanding footprint say with 1,000 grit than with 180 grit...thus more sand scratches and more teeth with 1,000). If chemical adhesion didn't occur by opening up a surface, would a paint company recommend using fine grits like 1,000 and even finer such as 1,500 on a clear blend on a sail panel? Also, I prep everything with 800 wet for base coat...some people swear that 400 grit gives better adhesion...I haven't had a peeler with 800 grit, and now that water born is out, PPG recommends sanding with 800 grit wet for base coat, why, enough of a grit to open the primer and because water born is so thin, 600 grit wet sand scratches have tendency to show...yet adhesion is not a problem.

Opening up primer by sanding allows the solvent to penetrate the primer, the solvents go through the primer, hit the metal and bounce back up. This also helps explain things such as solvent popping, poor adhesion of base coat if proper flash times aren't allowed and host of other scenario's that can effect paint.

I think we have all heard the term, paint needs to breath, the only way paint can breath is if it is porous to a degree. This is evident when a fresh paint job is waxed...the paint can't breath and eventually the clear or the color dies. Again, what we are doing by sanding is opening up the pores even further.

This how I've had it explained to me and it does make sense. One more note, I remember a prepper that did a color change on his car, every thing but under the hood, in the door jambs, inside the trunk and in behind the gas tank flap, he did however sand behind the flap. Within a year, the bottom lip on the inside of his gas tank flap didn't have any paint on it anymore.
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  #96 (permalink)  
Old 06-13-2013, 08:38 PM
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On another note, if reducers didn't enter a substrate to form chemical bond, why would some be harsher than others, why would there be a need for some reducers that if not used properly that can actually wrinkle an existing finish...if all that was left was mechanical adhesion...that's one problem that most of us have experienced. Reducers, acetone, varsol, gasoline are all solvents, Lacquer paint is cut with Lacquer thinner...what does lacquer thinner do to lacquer paint, it will take it off if to much is used, if the right amount is used it will penetrate the lacquer paint and again, give a chemical bond.

Ray
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  #97 (permalink)  
Old 06-13-2013, 09:02 PM
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Maybe acetone will do some damage,I don't know what all of this proves, this is sorta useless info. Maybe good for those who drive around with epoxy primer on their car.
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Old 06-13-2013, 09:10 PM
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Quote:
Originally Posted by 69 widetrack View Post
On another note, if reducers didn't enter a substrate to form chemical bond, why would some be harsher than others, why would there be a need for some reducers that if not used properly that can actually wrinkle an existing finish...if all that was left was mechanical adhesion...that's one problem that most of us have experienced. Reducers, acetone, varsol, gasoline are all solvents, Lacquer paint is cut with Lacquer thinner...what does lacquer thinner do to lacquer paint, it will take it off if to much is used, if the right amount is used it will penetrate the lacquer paint and again, give a chemical bond.

Ray
It all goes back to everything I said earlier about solvents. Solvents dissolve their respective substrate. If you add a lot, it dissolves it all, if you only add a little, it only dissolves a little bit. Eventually the solvent evaporates and you're left with your substrate. Add a teaspoon of water to a pile of sugar, the sugar gets wet, only some goes into solution. Add a gallon of water to sugar, the sugar disappears and goes into solution. When the water's evaporated, you're left with a pile of sugar. It's the same thing that's occurring with lacquer thinner. Solvents have varying degrees of harshness because it all depends on those four chemical properties I discussed earlier.

If the solvent can simultaneously solubilize two different substrates, maybe molecular cross-linking can occur between these two substrates, in which a 'chemical bond' would occur.

Last edited by Lizer; 06-13-2013 at 09:15 PM.
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Old 06-13-2013, 10:16 PM
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  #100 (permalink)  
Old 06-13-2013, 10:22 PM
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To dissolve, they need to permeate and that is my point...chemical adhesion, crosslinking, this is not about anybody driving around with epoxy Primer on their cars...it's about understanding how paint reacts. For those of you that feel that this is about anything else...thank god there are people that want to understand...or you might be driving around with less than epoxy primer.
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  #101 (permalink)  
Old 06-13-2013, 10:29 PM
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Ray, you really lost me there.
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  #102 (permalink)  
Old 06-13-2013, 10:37 PM
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All I'm saying is that if solvent dissolve, they need to get into the substrate, if they don't they can't dissolve. So would it be safe to assume that solvent enter the substrate easier when sanded than when not sanded?

ray
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  #103 (permalink)  
Old 06-13-2013, 10:44 PM
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Originally Posted by 69 widetrack View Post
All I'm saying is that if solvent dissolve, they need to get into the substrate, if they don't they can't dissolve. So would it be safe to assume that solvent enter the substrate easier when sanded than when not sanded?

ray
Sounds totally logical to me. Which is probably why they also say to sand paint first before adding a chemical stripper to it.
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  #104 (permalink)  
Old 06-13-2013, 10:56 PM
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Exactly, to open the pores in the paint and allow the stripper to fully work it's chemical reaction.

Reducers are made not only to thin paint, they are designed so that the reaction between the substrate is conducent with the fresh top coat and can cross link or form a chemical bond easier with a sanded substrate, because it's open it allows the chemical reaction to happen easier and faster....other wise, why wouldn't we use paint thinner to thin our paint, even water born reducer/water has active solvents in it in order to form the cross link. Why, if all we would need is mechanical adhesion would they need to put active solvents in the "water"/solvent in water born base coat?
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  #105 (permalink)  
Old 06-14-2013, 07:54 AM
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Quote:
Originally Posted by Lizer View Post
No I'm saying that what makes sense to me is there is only physical bond when a cured finish is sanded. I'm also saying that I don't know if there actually is any chemical bond at any point, unless more coats are added to a finish not completely cured so it is still susceptible to (dissolved by) the solvents in the newly added coats.

The chemistry and science of paint finishes fascinates me, and when I work on my car, I really try to apply the science I know with the art and skills I try to learn here and elsewhere. I just wish I knew more about the actual chemistry. What I'm saying in the above paragraph are just musings...educated guesses maybe, but don't accept them as the truth. I'm not a chemist.

Ok, I understand what you are saying, and I agree, IF the film is fully cured. IF the film is a 2k that is fully cured, no chemical bond, but if it isn't fully cured or if it's a 1K there is a bond, correct?

Brian
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