Enamel (or PU) over epoxy sealed plywood - How? Primer?

[Larry C]

Why so long for primer to cure? My estimate is:

It's an oil (alkyd) based primer which I now understand cures through an oxidative process. The epoxy coating on one side prevented access to oxygen. The second coat, applied before the first had cured, terminated access to oxygen from the other side. I think the second primer coat isn't completely impervious to oxygen migration, but one can imagine is slows it down an awful lot. Primer brushed edges adjacent un-primed areas only got one coat, right at the feathered edge; those have cured nicely. If it hasn't cured by the time I finish everything else, I'll just sand it all off and start the paint afresh.

BTW: An additional guess is that if the cabin hadn't been encapsulated in epoxy, the oxygen in the pore-space of the wood would have been available to drive the cure process, at least to some degree.

Not sure your analysis is the reason, but try searching boat building forums for epoxy/paint or varnish compatibility issues. This has been brought up several times. Also, most of the epoxy suppliers have recommendations regarding coatings that work (or don't) with their product.

I experienced non hardening paint over a 2 week cured epoxy/glass job on a boat. I tried cheaping out on the project because it was a restore job.
The well known paint never dried at all. This was a quality alkyd enamel from a Big Box. I ended up scraping off the the paint, using a scraper, scotchbrite, and solvent. It took quite a bit of effort, but once I resanded everything, it was all gone. I then applied Brightsides which dried hard enough to sand over night. The second coat made it look like a custom spray job.

The moral of the story is always, always pre-try your final finish before totally committing.

I think the issue of compatibility is from manufacturers constantly changing formulations to meet government requirements and, reducing costs, or having been bought out by some that is now making crap. What worked fine yesterday, may not work at all today.

As for Amine blush, I stopped using epoxy that produces blush a long time ago. However, I wet sand with water just to be sure before applying final finish. I have never had an issue with blush, but their is always a first time!

Larry C

[doug hodder]

I spray acrylic urethanes over straight epoxy resin all the time, no primer. I only use a primer so I can block sand it to see if there are low spots in the material. Like mentioned...check with your epoxy manufacturer for their recommendations as to what's compatible. Doug

[jmedclay]

[quote="Larry C"]
Not sure your analysis is the reason, but try searching boat building forums for epoxy/paint or varnish compatibility issues.

The moral of the story is always, always pre-try your final finish before totally committing.
Larry C[/quote]

Thanks for the information Larry. After some internet searching, I see what you mean. I won't go that route again. Live and learn. Meantime I'll finish wiring, fiberglassing and fettling-in the hatch. It won't get this primer! Then I'll build the steel trailer. If the primer hasn't cured by then I'll remove it and start fresh, with a different system - well tested. The thinnest primer edge areas are tough, like the same primer on bare wood, so I'm somewhat hopeful.

I haven't seen any blush with the Raka epoxy, so pretty happy there.

[afreegreek]

you don't need primer for good adhesion but fairing epoxy is much more labour that fairing primer.. it all depends on how fine a finish you want and how much work you're willing to do.. if you want a premium grade finish 320 grit is the coarsest you want to finish sand with.. it is always the best practice to use a compatible primer from the same manufacturer as the paint..

we use dye on the primer, usually blue aniline mixed with alcohol to stain the primer, then sand it off until none is left.. the blue will show divots and low spots even the best lighting will hide..

one of the most important tools in finish sanding is a good long block.. one of the best is Dura-Block and they're available at most auto body supply stores.. sanding with a small block or with an orbital will not give you the best results.. a perfect fairing is achieved by hand sanding with as long a block as possible.. no paint will give you a better finish than the surface it's applied to.. the glossier the finish the more important this becomes..

http://www.tcpglobal.com/autobodydepot/durablock.aspx

this photo is not primer, it is epoxy surfacing putty, this is the stage before primer is applied but the process will be repeated for the primer and again before the final coat of paint.


.

[afreegreek]

Why so long for primer to cure? My estimate is:

It's an oil (alkyd) based primer which I now understand cures through an oxidative process. The epoxy coating on one side prevented access to oxygen. The second coat, applied before the first had cured, terminated access to oxygen from the other side. I think the second primer coat isn't completely impervious to oxygen migration, but one can imagine is slows it down an awful lot. Primer brushed edges adjacent un-primed areas only got one coat, right at the feathered edge; those have cured nicely. If it hasn't cured by the time I finish everything else, I'll just sand it all off and start the paint afresh.

BTW: An additional guess is that if the cabin hadn't been encapsulated in epoxy, the oxygen in the pore-space of the wood would have been available to drive the cure process, at least to some degree.

Not sure your analysis is the reason, but try searching boat building forums for epoxy/paint or varnish compatibility issues. This has been brought up several times. Also, most of the epoxy suppliers have recommendations regarding coatings that work (or don't) with their product.

I experienced non hardening paint over a 2 week cured epoxy/glass job on a boat. I tried cheaping out on the project because it was a restore job.
The well known paint never dried at all. This was a quality alkyd enamel from a Big Box. I ended up scraping off the the paint, using a scraper, scotchbrite, and solvent. It took quite a bit of effort, but once I resanded everything, it was all gone. I then applied Brightsides which dried hard enough to sand over night. The second coat made it look like a custom spray job.

The moral of the story is always, always pre-try your final finish before totally committing.

I think the issue of compatibility is from manufacturers constantly changing formulations to meet government requirements and, reducing costs, or having been bought out by some that is now making crap. What worked fine yesterday, may not work at all today.

As for Amine blush, I stopped using epoxy that produces blush a long time ago. However, I wet sand with water just to be sure before applying final finish. I have never had an issue with blush, but their is always a first time!

Larry C

paint and primer have expiry dates.. many of the big box stores buy then sell old out of date products.. this is likely to be the problem with the product you bought.. could also have been improperly stored at one time or another..

[jmedclay]

Thanks for all of the information gang. I think I may get a slop-shot out of this paint exercise; the primer is slowly curing. It's getting tough and doesn't come up with a fingernail scratch anymore. Meantime, on the hatch, I added some deadwood for support of the tail and kitchen lights and am now fiberglassing the outer skin. The glass cloth is wrapped around the edges and the second epoxy application it tacking up. By the time I finish that and then mount and wire the lights I think I'll be able to paint again. Once that happens, a lot of elements will coalesce quickly - fan, doors, hatch, weather-stripping, windows. It should look like a functional cabin and kitchen pretty soon.

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