Chemical compatibility between SPI epoxy and partially cured moisture cured urethane

Andy S

Andy S

Promoted Users
I've got a number of pockets and partially hidden areas such as behind brackets that it's impossible to perfectly clean, and impossible to get a spray gun in there. So I'm going to brush in moisture cured urethane like Mastercoat or Eastwood Platinum. But there will be some overlap, maybe few inches where the SPI epoxy will go over the MCU.


So, I'm thinking about how the MCU cures, it starts out as cyanate terminated prepolymers, then these eventually turn into amines, and we get a urea linkage between these amine terminated polymers, -NH-CO-NH-.

Now with epoxy, I believe the epoxy resin usually contains the epoxide (oxirane) rings, and the hardener typically contains amines. And the general reaction is:

Epoxide+Amine -> Crosslinked epoxy network


I don't think exoxide reacts with cyanates, but during cure, there are active amine groups, so could the epoxide form a crosslinked network with the MCU polymers?

Don't know...

Maybe it's better to let the MCU fully cure , sand the visible areas, and just go for a mechanical bond???

Sorry, I'm a physicist, we geek out over this kind of chemistry
 
an mcu will not offer you any more protection than epoxy. that part where they claim its some kind of rust killer is all marketing bs. if the metal isnt clean and prepped then then rust will just fester under it and grow. i have not put epoxy over an mcu but per por15's tds when i read it maybe 15 years ago, you really cant put anything over it without some kind of adhesion promoter or tie coat. you wont find many people here if any that will recommend any kind of mcu. the only thing i would use it on is maybe my landscape trailer or something like that.
 
Mastercoat and POR are both MCU’s but they’re quite different. POR can’t be easily or safely top coated because it dries as a glossy black finish meant to be a final coat, whereas Mastercoat is aluminum pigmented and does function more as a primer in that it’s supposed to be top coated.

I’ve been using the Mastercoat silver for 15 years on specialty areas and for specialty things (I actually use it as an adhesive for emblems and if I want something aluminum colored. I also put it over an acid treated area first if I need to clean something up with acid first. Then I follow up with epoxy). I also use it on frames, suspension parts, undercarriage parts, etc, and anything that I can’t get clean enough for epoxy. I never use it directly over rust.

SPI epoxy is directly compatible with it. If sprayed within 24 hr it doesn’t need sanded (I’ll top coat it the very next day), however that’s also not in high humidity. If it’s been more than 24 hrs it needs to be scuffed by sanding. Reaction won’t be an issue, it’s the adhesion.

I’ve sprayed everything directly over the silver…basecoat, single stage, 2k enamel, epoxy, rattle can, latex house paint (in the case of a metal house door), it’s not very reactive at all. It sticks to a substrate very well and isn’t picky about what it sticks too. The biggest thing is just making sure you’ve sanded the surface or topcoat within 24 hours.

Does it offer better rust protection than epoxy? In my experience it does not. But the reason I like it for frames is because I then topcoat it with Mastercoat AG111 and that combination IS extremely robust, significantly more robust than epoxy alone. I’ve had that combination on two frames, my tractor, a horse trailer, and farm equipment. Things in high abuse situations with about 10 years of trial on it now. It holds up very, very well. As in nothing has started failing yet.
 
my limited experience with mastercoat is as Lizer, get any thing you want over it inside the window. that stuff when cured is near krazy glue and i dont any thing will adhere to it once cured. i use it in certain situations, but i would not substitute other brands for it. i dont like any thing eastwood makes as far as paint. i wouldnt use por 15 either. there is another commercial brand........is it alumacoat? i cant remember at the moment.
 
I’m using a low viscosity MCU (mastercoat / rust bullet ) in these weird pockets because it can be applied with a brush , has much lower surface tension than epoxy , is compatible with less than ideally prepared surfaces (like behind a bracket or in a wedge shapes pocket ) and simply wicks into seams .

Whereas epoxy is higher surface tension and is really designed to be atomized .

That’s why only pockets get MCU and anything visible that i can properly prep gets epoxy.

But there is going to be some overlap , so just checking on that .
 
Andy S said:
I’m using a low viscosity MCU (mastercoat / rust bullet ) in these weird pockets because it can be applied with a brush , has much lower surface tension than epoxy , is compatible with less than ideally prepared surfaces (like behind a bracket or in a wedge shapes pocket ) and simply wicks into seams .

Whereas epoxy is higher surface tension and is really designed to be atomized .

That’s why only pockets get MCU and anything visible that i can properly prep gets epoxy.

But there is going to be some overlap , so just checking on that .
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For stuff you're brushing on, you'd be much better off to use the orange label thicker stuff. The sprayable 'slim' (blue label) is pretty thin and would require a lot of coats brushing. It works well for spraying but doesn't offer near the rust protection as the thicker stuff. It takes at least 3 coats of the 'slim' to equal 2 coats of the regular. But I'm guessing you've already purchased it.

Also--put a piece of saran wrap over the top of the can before replacing the lid otherwise you're never getting the lid open again. And keep the lid open only for the minimal time possible. I open it long enough just to stir and then pour what I need into a secondary container, and put the lid back on.
 
Here’s what I mean by a 3 zone problem


This is looking into the door jamb of a Volvo P1800, this shows three different areas , (1) the outside and accessible areas , (2) inner areas that are impossible to remove every last bit of rust , but are cleaned with wire wheel and chemically, and (3) transition areas where they are accessible on one side and inaccessible on the other .

(1) The outer areas OBVIOUSLY get epoxy primer. (2) The inner partially hidden areas get a moisture cured urethane sprayed with wand or brush , like MasterCoat or Rust Bullet .

I still can’t figure out how the handle the transition areas .

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honestly from what i can see that looks clean enough for epoxy.......however, if you have used any kind of acid and are not 100% certain you have it neutralized, then use the mastercoat. read the instructions for the mastercoat, i believe you can put epoxy over it inside the window. as such, i would do it in stages, mastercoat first, then epoxy, but be sure to follow each sheet on window times. i want to say its 24 hours on MC but you better read the sheet. i have never used rust bullet so i cant say any thing about it. other wise you could tape and mask.
i would be way more worried about acid than i would about epoxy over MC
 
Treeslayer said:
honestly from what i can see that looks clean enough for epoxy.......however, if you have used any kind of acid and are not 100% certain you have it neutralized, then use the mastercoat. read the instructions for the mastercoat, i believe you can put epoxy over it inside the window. as such, i would do it in stages, mastercoat first, then epoxy, but be sure to follow each sheet on window times. i want to say its 24 hours on MC but you better read the sheet. i have never used rust bullet so i cant say any thing about it. other wise you could tape and mask.
i would be way more worried about acid than i would about epoxy over MC
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Yes I did use a phosphoric / citric mix inside the cowl area , the citric chelates and solubilizes the loose FePO4, then rinsed with a citric acid / sodium hydroxide , at pH of 6, this makes all 3 binding sites on the citric active so that it grabs the remaining loose iron oxide and iron phosphate and makes it soluble, then ammonia rinse to wash all that stuff out and keeping the pH above 8 to prevent flash rust , and a final alcohol rinse to remove remaining oils / waxes and aid in quick drying .
 
Treeslayer said:
yeah i was lost there lol. i guess thats why im leery of acid.
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Phosphoric is safe , but makes a mess if you’re not mindful of the chemistry

Now at work , I have my students do a lab under extreme supervision with hydrofluoric . That stuff is scary and without the antidote , one drop is lethal . So only used with full suits .
 
Getting ready to shoot this 1/2” tall, 3” deep gap between upper frame rail and fender , and this 6” deep wedge in the back is the fender with Eastwood rust encapsulator moisture cured urethane , and have a long reach wand for it .

In the main masked off area , I’ll shoot SPI epoxy .

These are deep gaps , and the Eastwood long reach wand is the only thing that gets in there.

I’ll follow up with compressed air blast to ensure no pooling or puddling to avoid solvent entrapment .


I spent weeks cleaning these areas , wire brushed , chemically cleaned , sand blasted , but still can’t get every minuscule spot

I have no idea if I’m using the right approach with this hybrid solution .



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You’re overthinking this. My process: I use Mastercoat metal prep if I need to (which is phosphoric acid, zinc phosphate, and a detergent. Wipe off any excess. Let it dry (about a day). Spray the Mastercoat directly over it, within 24 hours topcoat with epoxy.

No neutralization step is need, no NaOH required, no ammonia, just wipe off the excess and go. WGR before anything is applied.

I’m trying to figure out what’s going on with the masked off sections.
 
Lizer said:
You’re overthinking this. My process: I use Mastercoat metal prep if I need to (which is phosphoric acid, zinc phosphate, and a detergent. Wipe off any excess. Let it dry (about a day). Spray the Mastercoat directly over it, within 24 hours topcoat with epoxy.

No neutralization step is need, no NaOH required, no ammonia, just wipe off the excess and go. WGR before anything is applied.

I’m trying to figure out what’s going on with the masked off sections.
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Acids and low ph cause a lot of problems for urethanes and epoxies, so that’s why I’m very thorough with cleaning .

Final step was alcohol / acetone on paper towel on long forceps that I dragged through there . Impressive how much junk that lifted .

The masked sections are these channels where I’ll be shooting the Eastwood MCU, so masking off the main surface which will get epoxy . There’s a 1/2” wide , 3” deep channel that’s between the upper unibody frame rail and the fender , and a 6” deep wedge at the back of the fender, and that top of the upper frame becomes the floor of the cowl under the windshield , so it’s like a big horseshoe shaped shelf all around the engine bay .
 
you have probably already applied the eastwood...........i had bad luck with that stuff. i wouldnt put it even in the same category with mastercoat.
good luck to you sir, i hope it works out. perhaps your prep work is the key to it lasting.
 
Treeslayer said:
you have probably already applied the eastwood...........i had bad luck with that stuff. i wouldnt put it even in the same category with mastercoat.
good luck to you sir, i hope it works out. perhaps your prep work is the key to it lasting.
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Here is some of the overspray / drainage of the Eastwood internal frame coating , and me shoving a pick underneath it . The thing that really impresses me is that the paint tares , but it does not lift or peel. POR15, presumably because it’s super high in ratio of hard polymers dries like glass and flakes off in sheets . This is about 3 weeks after painting and fully cured . This is the backside with no prep, well I cleaned this 6 months ago with citric / Zep, but it’s been exposed and dirty , and eventually got flash rust from humidity .

Main reason I’m using the Eastwood here is it has the wand nozzle to get into those cavities .

Here’s a video I made of applying the Eastwood in the frame rails :


Here’s a pic of what the overspray looks like picked

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