What's the finest grit on sheet metal you can spray epoxy over?
- Thread starter MikeS
- Start date
O
Outlaw
If im using filler over the epoxy, I always felt I needed more than 80 grit, but I am not experienced like a lot of the members here. Im guessing filler likes more than an 80 grit scratch, but if your applying epoxy first, the chemical bond makes up for a finer grit? I am also under the assumption the chemical bond is superior to the mechanical bond but again, thats just how I have interpreted what I have read on this forum. I dont always interpret thing correctly, lol.
Chris_Hamilton
Trying to be the best me, I can be
The adhesive bond of epoxy to the filler far outweighs the mechanical bond. The main reason for 80 grit DA on metal is to give the epoxy a solid mechanical bond to the metal (in addition to the adhesive bond of the epoxy to the metal). FIller doesn't need anything coarser than 80 grit.Outlaw said:
O
Outlaw
Thanks Chris, my learning curve never ends.
S
Senile Old Fart
I have often wondered what might be the perfect anchor pattern.
Surface area is a factor with grip. the more area the more grip.
At what point does the anchor pattern become to course of fine to decrease the amount of surface area?
Secondly, how many microns of surface area does a molecule of the coating being applied need to be?
If the valley of the pattern is to narrow the molecule will only bridge the surface and leave a void under it.
If the peaks of the pattern are to far apart the molecules will not be able to fill the valley in one coating.
To say sand with a given grit is the solution is only partially correct.
Look at the pattern produced in the first 30 seconds of sanding with a new abrasive, Compare that to the pattern produced after 3 minutes of sanding with that same abrasive.
The pattern will have gotten finer the longer the abrasive was used.
Now at what point do you change to a new disc?
Most all agree that the oem e-coat is one of the best "stuck" primers .
The base metal was washed and probably etched, then neutralized and washed again before application of the e-coat.
I wish I knew what the anchor pattern was on that new sheet metal right before the e-coat is applied.
My instinct says it is much finer than 80 grit da scratches.
I was involved in the maintenance of a coating line that applied either epoxy or polyester coatings to aluminum. That metal was subjected to the wash - etch- neutralize - rinse -rinse method then it had the coating applied and then furnace dried. I never thought at that time to ask a product engineer what the anchor pattern was.
The coatings we applied were all in a non contact roll method, vs dip or spray, but I don't think the type of application would make a great difference in the anchor pattern requirement.
Sorry for the long post, just wanted to see what others think about the "mechanics of adhesion"
Surface area is a factor with grip. the more area the more grip.
At what point does the anchor pattern become to course of fine to decrease the amount of surface area?
Secondly, how many microns of surface area does a molecule of the coating being applied need to be?
If the valley of the pattern is to narrow the molecule will only bridge the surface and leave a void under it.
If the peaks of the pattern are to far apart the molecules will not be able to fill the valley in one coating.
To say sand with a given grit is the solution is only partially correct.
Look at the pattern produced in the first 30 seconds of sanding with a new abrasive, Compare that to the pattern produced after 3 minutes of sanding with that same abrasive.
The pattern will have gotten finer the longer the abrasive was used.
Now at what point do you change to a new disc?
Most all agree that the oem e-coat is one of the best "stuck" primers .
The base metal was washed and probably etched, then neutralized and washed again before application of the e-coat.
I wish I knew what the anchor pattern was on that new sheet metal right before the e-coat is applied.
My instinct says it is much finer than 80 grit da scratches.
I was involved in the maintenance of a coating line that applied either epoxy or polyester coatings to aluminum. That metal was subjected to the wash - etch- neutralize - rinse -rinse method then it had the coating applied and then furnace dried. I never thought at that time to ask a product engineer what the anchor pattern was.
The coatings we applied were all in a non contact roll method, vs dip or spray, but I don't think the type of application would make a great difference in the anchor pattern requirement.
Sorry for the long post, just wanted to see what others think about the "mechanics of adhesion"
O
Outlaw
One thing new I noticed coming up several times in this thread is the reference to "80 DA scratches". I always hand block everything with a criss cross pattern before applying another level. Never took into consideration the difference between hand blocking and DA. Also, I wish Barry would give us his personal opinion on how he would rate mechanical adhesion to chemical adhesion. I agree, there is no sign I can see on a oem panel that suggests there was an 80 grit to apply the primer on. Maybe its a chemical bond process?
crashtech
Combo Man & Mod
Adhesion is a weird science, I tried to learn something about it a while back but it just made my brain hurt.
But the point about sanding with dull sandpaper is the the one anybody reading this should take away with them. Don't be stingy with the paper! You have to have a sharp cut or you are just polishing/smearing metal around, the worst example of this is a wire brush. Fresh 80 sandpaper should leave the surface feeling a bit rough.
One of the best anchor patterns is good sharp media blast, but metal etching can leave a nice microscopic anchor pattern, and some etching processes leave a microcrystalline layer of phosphate or the like that can provide good interlocking properties. It's the last two that OEMs use on their parts, physically sanding them would be WAY too labor intensive.
But the point about sanding with dull sandpaper is the the one anybody reading this should take away with them. Don't be stingy with the paper! You have to have a sharp cut or you are just polishing/smearing metal around, the worst example of this is a wire brush. Fresh 80 sandpaper should leave the surface feeling a bit rough.
One of the best anchor patterns is good sharp media blast, but metal etching can leave a nice microscopic anchor pattern, and some etching processes leave a microcrystalline layer of phosphate or the like that can provide good interlocking properties. It's the last two that OEMs use on their parts, physically sanding them would be WAY too labor intensive.