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

[jmedclay]

I've sealed my TD plywood with several coats of thinned Raka epoxy. Now I need to paint it. Alkyd enamel sounds like an economical product to use so I'm interested in the details of how you do it.

I'm pretty sure that the epoxy needs to cure for a week or so, then be cleaned and sanded, but is a primer needed after that, and before the enamel? If so, what type should be used? Or, is it perfectly reasonable to apply the enamel to the sanded epoxy surface?

Same question if a two part polyurethane is used instead.

Thanks
John

[Ageless]

With wood; the primer acts as a sealer which you have already done with the epoxy. Give it a light sanding for bonding and begin painting

[wannabefree]

Alkyd should do fine. Sand to 180 grit so it has some tooth to bond to. I would recommend letting your epoxy cure for a month or more. While it cures, make a test panel. Let the test panel cure a week, sand, and paint as you plan. let that cure a week, then throw gravel at it, scratch it with your fingernail and a screwdriver. Then, if you're happy with that, paint your trailer. Sounds like a lot of work, but it's faster than doing it over if the results aren't what you expect.

[Ageless]

Wannabe; what you have described in manufacturing is a 'test coupon'

[Juneaudave]

Just to be different...even with epoxy, I tend to use a primer before topcoat and give that primer a good sanding with a fairing board. That assures that I know if there are low spots and I figure that it won't hurt. On the last boat, I used Interlux Brightsides Marine and Interlux Pre-cote on fiberglass and applied it using a "roll and tip" method.

Having said that, a primer might not be necessary for epoxy. but if you use a primer, don't cheap out. Your paint isn't going to stick any better than the primer.

Just my thoughts...Dave

[starleen2]

Make sure you do a proper wipe down with a light, non aggressive solvent solvent after the sanding. Depending on the epoxy used, it might "blush" with a slight waxy coat on top. This coat is usually sanded off and then the final coat applied. if it is not removed, the paint will not adhere to that particular area. I learned this the first time I used fiberglass.

[planovet]

I used 2 coats of CPES, 3 coats of primer and 5 coats of topcoat. For the topcoat I used an acrylic enamel with a hardener. It was a lot of work sanding between each coat but the results are worth it. A lot of people think it's fiberglass or something like it. The more work you spend now will mean less work down the road. Good luck!

[jmedclay]

Lots of good tips - thanks very much. I think I'll end up using alkyd enamel over primer, after cleaning and skuffing the "cured for a while epoxy". The guy at Raka said a week would be OK, but it will probably be closer to two.

[Nosty]

OK, now I'm curious? Or is that confused? Doesn't Rust-Oleum make an Alkyd Enamel? Are you saying that, after the resin cures, it's OK to just coat with Rust-Oleum, (or another brand), Alkyd Enamel? If that's so, that sure is a load off the wallet.

Brian

[afreegreek]

there is only one reason to use primer and that's because it's faster to build and sand smooth than epoxy (cheaper too) say you're using exterior grade fir plywood with patches in it. you could use epoxy to seal the wood then use high build primer over that to build up a film think enough to sand fair. you could do the same thing with epoxy alone but it takes longer, is more expensive and much harder to sand. as far as sticking goes there is little to no difference. if you have an epoxy coated surface you are happy with just go ahead and paint it.

here's some info on alkyds from Wiki

Alkyd coatings are typically sold in three classes; Long, Medium, and Short. These terms represent the relative fraction of drying oil component in the resin. Long oil alkyds have a high percentage of drying oil content and are generally sold as medium duty coatings for the consumer market. Medium oil alkyds have less drying oil and have a higher percentage of large molecular weight polyester backbone. They dry slower and are employed as high gloss coatings and wood finishes. At the bottom end are short oil alkyds where the percentage of drying oil is very low in relation to the base polyester polymer or backbone chain. These coatings will not air dry or harden unless heated. Short oil alkyds are employed as baking enamels for finished metal products.
Because the major components of an alkyd coating, i.e. fatty acids and triglyceride oils, are derived from low cost renewable resources this has kept the cost of alkyd coatings very low despite ever increasing cost of petroleum which is the predominant raw material source of most other coatings such as vinyls, acrylics, epoxies, and polyurethanes. Typical sources of drying oils for alkyd coatings are sunflower oil, safflower oil, walnut oil,soybean oil, fish oil, corn oil, and tall oil (resinous oil by-product from pulp and paper manufacturing).

[jmedclay]

They say the first thing that goes away with age is memory; I forget what the second thing is. Somewhat embarrassing, this, but if it saves someone else the trouble I've gone through then it's worth it.

1) I forgot I asked these questions or even started this thread!
2) I know to use a test area for pretty much anything new.
3) I used the alkyd primer, let it dry the 4 hours on the label and re-coated - BIG MISTAKE
3a) didn't need it
3b) didn't let it cure
4) Dry to touch but not adequately cured... I figured that out later. Applied second coat, which sealed the first from oxygen and has slowed the cure to a crawl. Still didn't know all of that.
5) Next day cut in the door jambs with a Benj Moore latex formulation (new low VOC chemistry) that I hadn't used on the bottom. It didn't wet out, acted a bit like water on Rain-X - not that extreme but it clearly didn't have an affinity for the primer.
6) Another coat on the jambs - maybe the first coat just acts weird (not).
7) Figured out I had problems - paint not cured, not hiding well, just not working.
7) Days later I did a heck of a lot of scraping that was tedious and time consuming (the door jambs - wanted to do them first so I could apply the weather stripping and final install the doors).

Lessons:
1) Stop, don't rush head long into a new trade area: Remember previous posts, and advice given. (I did sand and otherwise prepare well)
2) Always test everything on a coupon, or at least a small flat workpiece area, so recovery is far easier and faster, if necessary.
3) If using an oil base paint, any paint that cures by exposure to atmosphere I suppose, be certain it's cured properly before applying subsequent coats.
4) If you had good luck with a particular paint on the trailer underside (I did, very tough, hard to sand, very pleased with it) remember that detail, ask for it, accept no substitutes....or test them off line for more time that might appear necessary.

I'm finishing up the hatch electrics while the first primer coat continues to cure (I think) very slowly. It's been two or three weeks since it went on, it is harder and seems to be curing but at this rate I'm thinking it will be several more weeks at least. The good news is that I can finish pretty much everything else in the interim...and the flat surfaces of the trailer are a heck of a lot easier to sand than the door jambs, if need be.

[Larry C]

Just to be different...even with epoxy, I tend to use a primer before topcoat and give that primer a good sanding with a fairing board. That assures that I know if there are low spots and I figure that it won't hurt. On the last boat, I used Interlux Brightsides Marine and Interlux Pre-cote on fiberglass and applied it using a "roll and tip" method.

Having said that, a primer might not be necessary for epoxy. but if you use a primer, don't cheap out. Your paint isn't going to stick any better than the primer.

Just my thoughts...Dave

+1 on Brightsides!!
If you want a paint finish that looks like it was sprayed on, this marine paint is a great choice. Interlux has the primer for using Brightsides over fiberglass, but other than what Dave mentioned he used it for, I don't see that it's necessary. I have just painted over the epoxy/glass with fantastic results.
The roll and tip method works great with this self leveling paint. This paint as anything with "marine" in it's name is expensive. However, it's a proven performer with lots of pigment, so 2 coats is usually all that is needed.
IMO Cheaper paints from Big Box stores just don't give the same results, and some are not compatible with epoxy, and won't dry properly. Don't ask how I know this.


Larry C

[schaney]

After your epoxy has cured the first step should be a scrub down with clean water and a scotch-brite pad for removing blush. No need to use solvents, water works best. On being fully cured, safe to top coat, check with the manufacture. On West Systems depending on temp, 2-7 days.

[Oldragbaggers]

+2 on the Interlux Brightsides. We've used it on every boat we've ever owned. It's tough, it's beautiful (very nice gloss and great color), it applies easily enough with the roll and tip method (just remember to keep it thinned properly) and it works great on fiberglass with no primer needed.

+1 on the advice for cleaning blush. Solvent is not necessary and is not the proper method for removal of blush. Good old fashioned soap and water work best. Clean before you sand or you're just grinding the blush into the surface.

Question...why is it taking so long for your primer to cure? What kind of primer is it?

[jmedclay]

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.