Do you still get a chemical bond after sanding ......

MJM

MJM

Promoted Users
Example:
1. spray two coats of epoxy
2. 24 to 48 hours later sanded with 180 grit
3. 24 to 48 hours later, spray either 2k or polyester primer over the sanded epoxy

Is this a chemical and mechanical bond for the 2k or polyester primer?

If so, is this chemical bond as good as if you hadn't sanded the epoxy and applied 2k or polyester 24 to 48 hours later?
 
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I never sand within around a 7 day window IF I have kept it out of the sun. Only sand to put on another coat of epoxy to get another 7 day window.

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It is likely still a chemical bond that is just as good plus a bit of mechanical. When I talked to Barry about leaving mine outside and if can just sand it for a mechanical bond he did not want me to do that... he wanted only the chemical but sanding and shooting another coat of epoxy before the filler.
 
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JimKueneman said:
I never sand within around a 7 day window IF I have kept it out of the sun. Only sand to put on another coat of epoxy to get another 7 day window.

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Yes i understand that is the standard operating procedure for SPI epoxy, not refuting that method.

I believe It was Shine who said he sands spi epoxy even if it's within the re-coat window before spraying another material over the epoxy. Hence my reason for asking if it's still a chemical bond if the epoxy has been sanded 24 to 48 hours after spraying the epoxy, then spraying another material over the sanded epoxy?
 
MJM said:
Yes i understand that is the standard operating procedure for SPI epoxy, not refuting that method.

I believe It was Shine who said he sands spi epoxy even if it's within the re-coat window before spraying another material over the epoxy. Hence my reason for asking if it's still a chemical bond if the epoxy has been sanded within the 24 to 48 hours, then spraying another material over the sanded epoxy?
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I can't see how that would change the chemical bond... the epoxy is not cured yet.. though I bet it does cut down on the window by sanding since it opens up the epoxy so it can cure faster.. likely only Barry can give a definitive answer.
 
JimKueneman said:
I can't see how that would change the chemical bond... the epoxy is not cured yet.. though I bet it does cut down on the window by sanding since it opens up the epoxy so it can cure faster.. likely only Barry can give a definitive answer.
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That was my thinking also but, maybe Barry will chime in and share his thoughts on the subject at hand. Will the chemical adhesion be as good sanding fresh epoxy 24 hours later before spraying another material.
 
My reason for asking is............every time I apply another material, it's always applied over a blocked "flat" sanded surface. This way I'm safe guarding myself from waves on a panel before applying another material.
 
Barry has said to me the chemical bond of unsanded in the window epoxy is all you need. That was good enough for me. I don't ever sand it before filler.
 
Chris_Hamilton said:
Barry has said to me the chemical bond of unsanded in the window epoxy is all you need. That was good enough for me. I don't ever sand it before filler.
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Like I already stated, I'm not disputing the standard operating procedure for SPI epoxy.

I inquired about this because I personally like to have a flat sanded surface before applying another material on top. Is it necessary, no.....does it hurt anything, no. It's more of this is how "I" do it, and this is the reason why.

No one is going to make me believe that someone, anyone, can lay down a coat or two of paint and it lays down as smooth as glass, no way that's happening. I don't care how good someone is at spraying, especially primers. So a person sprays two coats of epoxy, waits 24-48 hours and then sprays 2k or a polyester primer. I get that 2k or polyester primer is now that primer that will be blocked flat but, it's been sprayed over a surface that wasn't flat to begin with. I can see why break thru's happen more often doing it that way because of the uneven surface.

For me personally, I'm just trying to minimize error ( in this case, waves) in the final product, because once I get to cut and buff, I know the urethane wave is from the last product I sprayed, clear or singe stage, not the subsequent material below it. It's been ruled out by sanding each layer flat.
 
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MJM said:
Like I already stated, I'm not disputing the standard operating procedure for SPI epoxy.

I inquired about this because I personally like to have a flat sanded surface before applying another material on top. Is it necessary, no.....does it hurt anything, no. It's more of this is how "I" do it, and this is the reason why.

No one is going to make me believe that someone, anyone, can lay down a coat or two of paint and it lays down as smooth as glass, no way that's happening. I don't care how good someone is at spraying, especially primers. So a person sprays two coats of epoxy, waits 24-48 hours and then sprays 2k or a polyester primer. I get that 2k or polyester primer is now that primer that will be blocked flat but, it's been sprayed over a surface that wasn't flat to begin with. I can see why break thru's happen more often doing it that way because of the uneven surface.

For me personally, I'm just trying to minimize error ( in this case, waves) in the final product, because once I get to cut and buff, I know the urethane wave is from the last product I sprayed, clear or singe stage, not the subsequent material below it. It's been ruled out by sanding each layer flat.
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If it makes you feel better that's great. Not telling you to not do it, but the reality is with epoxy within the 7 day window it's not really "better" or "safer". The chemical bond is all you need and adding mechanical scratches doesn't improve the adhesion. The chemical bond is so strong that it is superior to what the mechanical bond would be. It's not a 2+2=4 type situation. The chemical bond is so much stronger than the mechanical, it outweighs it. So in reality it would be 2 (chem)+1 (mech) = 2. Hope that makes sense. This is all according to Barry.
 
Think of it another way. The mechanical bond alone is not as strong as the chemical bond. Therefore adding the mechanical scratches to the epoxy doesn't increase the adhesion as the mechanical bond is less than the chemical bond. Only way to increase the adhesion would be to add a type of bond that is stronger than the chemical bond between in the window epoxy and body filler.

Like I said if it makes you feel better go for it. But IMO it's a duplication of effort and time is money.
 
Well I'm glad to hear Chris that you feel adding a sanded scratch to epoxy 24-48 hours after epoxy application will not effect the chemical adhesion of the epoxy for the next added material, whether it be filler, 2k or polyester primer if done within the 7 day window.

I like to use 48 hours as my window for applying another material on top of the epoxy.
 
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Chris_Hamilton said:
Think of it another way. The mechanical bond alone is not as strong as the chemical bond. Therefore adding the mechanical scratches to the epoxy doesn't increase the adhesion as the mechanical bond is less than the chemical bond. Only way to increase the adhesion would be to add a type of bond that is stronger than the chemical bond between in the window epoxy and body filler.

Like I said if it makes you feel better go for it. But IMO it's a duplication of effort and time is money.
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Using the theme song from the TV show COPS, here's my next reply......

Flat panels..... flat panels, whatcha going to do to eliminate waves by not sanding........flat panels flat panels :)
 
Being a pragmatist by nature, I spread a little evercoat on an unsanded SPI epoxy coated piece of old quarter panel to do the very non scientific bend and bang test on “ out of time/heat exposure window.
It surpassed all my expectations.
Your results may differ
 
I've dropped clumps of wet filler on the garage floor, and a week later having to use a hammer and chise to get it off.

Then after cleaning it up I look over at all the boxes of masking paper I have. Such is life......
 
Depending on the process in use, I will almost always sand epoxy, not especially for adhesion, although I do think it can only help, but for another much more important reason. The typical use of epoxy is over either bodywork or panels that may possibly need more bodywork. Blocking the epoxy is a great way to tell if you are REALLY done with all your filler/polyester putty work. If you are going to poly primer this step can certainly be skipped, but heading into 2K urethane it's much more important, since best practice it to NOT apply poly putty on top of urethane primer.
 
MJM said:
My reason for asking is............every time I apply another material, it's always applied over a blocked "flat" sanded surface. This way I'm safe guarding myself from waves on a panel before applying another material.
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I've also blocked before the window is over because I want it to be flat.
 
crashtech said:
Depending on the process in use, I will almost always sand epoxy, not especially for adhesion, although I do think it can only help, but for another much more important reason. The typical use of epoxy is over either bodywork or panels that may possibly need more bodywork. Blocking the epoxy is a great way to tell if you are REALLY done with all your filler/polyester putty work. If you are going to poly primer this step can certainly be skipped, but heading into 2K urethane it's much more important, since best practice it to NOT apply poly putty on top of urethane primer.
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My replies were strictly speaking to the fallacy that sanding the epoxy where filler will be applied will give better adhesion. I wasn't speaking to any other reason.
 
MJM said:
Using the theme song from the TV show COPS, here's my next reply......

Flat panels..... flat panels, whatcha going to do to eliminate waves by not sanding........flat panels flat panels :)
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I was only addressing the false notion that sanding the epoxy would somehow enhance or strengthen the bond between filler and epoxy.
 
The problem is if you’re blocking your epoxy flat then you’re removing mil build before the following coats of 2k or poly. Mils of epoxy is where your corrosion protection comes from so I leave as much intact as possible. And if you block some off then you should spray more, except now a cycle has started…

2K or poly will more than correct any possible wave from epoxy.

I only sand epoxy light for the sake of finding low spots to bump them. It’s ok to throw stuff on it the next day unless it’s poly.
 
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