adhesion questions

danp76

Oldtimer
Just to confirm:

Scenario 1: Month+ cured 2k urethane primer sanded 400-600 scratch.

Scenario 2: Month+ cured epoxy primer sanded 400-600 scratch

activated base coat may be applied over cured sanded urethane, but not cured sanded epoxy, both with the same scratch size? ***** both receiving a "new scratch" prior to applying base coat, to refresh scratch. Why is this?

Scenario 3: base coat applied over epoxy within 7 day window, but sanded with 400-600 scratch Scenario 4: base coat applied over urethane sealer with proper flash times Is the chemical bond between basecoat and epoxy or urethane better, or is the mechanical scratch a better bond? Some techs like wet on wet, others rely on mechanical adhesion, what do the test show for best bond/adhesion? Which "scenario" creates optimum results, and is there a measurable difference in adhesion?
 
Here is best example of what Jim is saying.
How many production shops over the years have scuffed a new factory ecoated panel and based?
Probably most but they never do it long and stop when the first car comes back in 6 months loaded with stone chips.

If all things are perfect, i have wet sanded epoxy up to two weeks and based but all storage and conditions were perfect. And testing showed no difference in adhesion but that is pushing it.
 
i will add this as well. i think it depends on the base you are using. i could sand epoxy, any epoxy, not just spi, that was 3 days old, base it then mask off for some graphics. when i peel the fineline the base base will pull off the epoxy every time BUT that was using diamont or hok which are both higher polyester bases. using spi bases i never had this issue.
 
So, why does this happen? Why can base be applied over sanded urethane, but not sanded epoxy? Both being several weeks old.... And what yields the best adhesion, my guess if fresh epoxy, but why? Why isn't there a mechanical bond on sanded epoxy, beyond 36hrs?
 
Exactly Jim, heavy polyester type base do not have the same adheision of a polyol base .
 
Dan, epoxy is so chemical resistant, atleast good ones, that the base wont bite into it at all. Think about this...you can sand utethane, epoxy and metal all with 80 grit and base right on them. Even though there is a tooth in the metal the base will peel right off. Epoxy is next on the list and urethane will bond no problem
 
So, after 36hrs epoxy should be recoated before applying basecoat? I used to do it up to 7 days and haven't had any issues.
my first coat of base is a reduced coat of epoxy .

Shine, how long do you wait to apply first coat of base, after you apply your first coat of epoxy as a sealer? Are you mixing 1:1:1?

How much adhesion/bond is lost by applying basecoat over sanded urethane primer? How much better is fresh epoxy? Is there a measurable difference?
 
i have never had an adhesion issue with base on any of the spi urethane primers when its sanded no matter how long it has sat. for epoxy, if you follow shine's advice which is how i do it as well then you never need to worry about anything. you will always have the best adhesion no matter what. just a single coat of epoxy mixed 1:1:1 15 min before you apply your base and your good to go.
 
Jim, 1:1:1, you can base in as little as 15 minutes? That's good to know. I was giving it minimum 30 minutes and more like 60 minutes most of the time.
 
How much better is epoxy adhesion compared to urethane adhesion? Is urethane adhesion better wet on wet? I would prefer to spray over a dried finish, less solvent for dye back and less mils and chance of dust etc. I would think spraying over wet epoxy, being a slow product would create additional dye back....
 
How much better is epoxy adhesion compared to urethane adhesion? Is urethane adhesion better wet on wet? I would prefer to spray over a dried finish, less solvent for dye back and less mils and chance of dust etc. I would think spraying over wet epoxy, being a slow product would create additional dye back....

If you let the reduced 1:1:1 epoxy flash off enough you won’t have that much solvent left behind, you spray it thin and the mil build should be pretty low as well.

I could swear that in some cases I have noticed better hold out, as if that single layer of epoxy was acting like a solvent barrier for the subsequent coats of base and paint jobs stay glossier.
 
One reduced coat of epoxy is so thin that it can be based over as soon as it flashes dull. It should be treated like its your first coat of base. You wont have any more solvent trapping that you would with a flashed coat of base. Epoxy may be a slow product but that is just the cure rate, not the flash rate.
 
Is there a scientific way of measuring adhesion difference between urethane vs epoxy for applying base coat over top? That is, what test have proven epoxy is better, and by how much?
 
Jim, 1:1:1, you can base in as little as 15 minutes? That's good to know. I was giving it minimum 30 minutes and more like 60 minutes most of the time.

It should be noted that no attempt should be made to reach full coverage. If you spray the epoxy just a little too heavy, you'll have big problems. If in doubt, reduce the epoxy less and wait longer before base.

Edit: I'm sure Chris would know this. I'm trying to help out any less experienced painters that might be reading this.
 
i have never had an adhesion issue with base on any of the spi urethane primers when its sanded no matter how long it has sat. for epoxy, if you follow shine's advice which is how i do it as well then you never need to worry about anything. you will always have the best adhesion no matter what. just a single coat of epoxy mixed 1:1:1 15 min before you apply your base and your good to go.

Jim,
What if the car is red with gray primers, will the single reduced coat of epoxy cover enough to avoid the different colors underneath base?
 
at 1:1:1 epoxy isnt going to cover. its there for adhesion only. its going to cover like a single coat of base.
 
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