Best time to apply anti-fouling paint after applying epoxy barrier primer?

[tworail]

Originally Posted by Ryban

Reviving an old thread for my sake here, but, what exactly is the different between epoxy barrier coats, epoxy primers, and straight-up epoxy? The internet seems mostly devoid of technical information, and mostly full of opinions.

As I understand it, epoxy barrier coats are two-part epoxy resins with high solids content (often proprietary) to improve moisture resistance. This also gives them their opaqueness.

Epoxy primers -- I really can only seem to find what they're for, but not what they are.

Why would the thumb-tack overcoat rule for antifouling apply to an epoxy barrier coat or an epoxy primer, but not to a straight-up epoxy?

I'm creating my own epoxy "barrier coat" with an epoxy resin, pigment, and graphite powder, and am in doubt about when to apply my anti-fouling after the final coat of epoxy.

Normal two part epoxy systems like West produce a 'blush' which occurs as the epoxy cures and must be washed off the surface before anything else is done. And if adding additional coats, must be sanded first to provide a physical bond otherwise the anti-fouling won't stick.

The two part barrier coat specific epoxies do not blush, and as the manufacturers recommendations say you can 'hot coat' the anti-fouling paint before the epoxy cures to establish a chemical bond between the barrier coat and the anti-fouling paint.

Interesting idea on the homebrew application - I think this would work but you would need to wash off any blush after each application once it fully cures. And you should be able to hot coat that as well to build up the necessary thickness. But you must wash and then sand the entire hull before applying the anti-fouling, so that is alot of extra work that could be avoided by using a conventional barrier coat system. Depending on the epoxy total cure time you still may achieve a chemical bond but I am not sure on that part, it would be good to hear from others that have used expoy resin systems to make their own barrier coat and skipped the whole Interlux/Pettit process.

In short, if you were using Interlux for example:
Sand off all existing paint down to barrier coat (if there is one) or to gelcoat. Apply barrier coats up to 5x which you can hot coat (thumb print test) then apply the necessary amount of anti-fouling (2-3x) which can also be hot coated to provide a chemical bond between the products.

I just went through this on my boat earlier this year and YES it was confusing also because I was using VC-17 which according to Interlux can't be hot coated onto the barrier coat but I got conflicting info from the dealer on that part.

Can't say much on the epoxy primer part, but generally if a product calls for a primer, you should use it. Primers seem to be used more for topsides applications as opposed to bottom side.

Also you may already have a good barrier coat on the boat already - you won't know until you haul out. Then there is system compatibility, you would have to know what barrier coat was used before to ensure it was compatible with your chosen bottom paint, etc etc.

Agreed, it can all be very confusing!!

Really enjoy your videos BTW, looking forward to the next instalment.

[Ryban]

Thanks for the reply, and I'm glad you're enjoying our adventure

From the West System instructions on using their product as a barrier coat, they advise over-coating the epoxies in one day, to avoid having to sand between coats -- so presumably the amine blushing that occurs either does not interfere with other coats of epoxy, or the blushing only occurs after a certain period of time, which must be longer than the "tacky" period.

However, like I mentioned, they do advise that the final coat be allowed to cure overnight, washed, and sanded before finishing. Between posting my question, and posting this reply, I contacted them and asked about this. The response I got was that though some users do apply their anti-fouling wet-on-wet, results have been mixed, so they always suggest allowing the resin to cure.

In any case, Tarka is a small boat, and a quick rinse and scrub, and light sanding sounds downright leisurely compared to the week of scraping we just went through.

I'd still like to know why barrier coats, like Interprotect, avoid the issue of amine blushing -- but that just might be my curiosity. I think I'd just respect the marine coating and paint industry a lot more of they sold the science behind their products rather than making blanket claims as to why product X is the best choice for the job.

I did make a test sample with a piece of wood I coated with a layer of epoxy, and I then applied a coat of the anti-fouling after the epoxy became tacky. As of tonight, the adhesion seems good, with little or no visible adverse effects, but we'll do some tests tomorrow once it really sets up.

[tworail]

Great, sounds like you have the right application formula for the epoxy and that makes sense - the blush normally won't occur until fully cured.

Not all epoxy systems produce amine blush, I believe either MAS and/or East Systems (and possibly others) do not blush either, so it could just be their chemical composition is slightly different. The barrier coat companies must be using a formula like this. I've gone through almost a gallon of East System epoxy this year and haven't noticed any blushing after cure on my projects.

Just like West though all these paint companies have tech hotlines with actual tech people answering the phone where you can ask questions - whether a customer or not, so it's a good resource.

This is an older article (2000-2001??), incidentally, ranks East as 'bad' for blushing but I never noticed it so it's possible the recipe has changed or the conditions I was working in reduced or eliminated the blush.

Epoxy test results from six epoxy types

[Pauls]

Some epoxies blush, some do not. Here is a link to Progressive Epoxy Polymers website page addressing blush:

Professional Marine Epoxy Resin / NON BLUSHING! /

[Ryban]

Thanks to both of you, this is good information.

Though I was skeptical at first, for my specific application (non-structural, barrier coat only), that there would be any important differences between a high-quality epoxy, and something "off-brand", I'm finding that to be wrong.

This is going a bit off topic, but my hope was to use the local resin ($75/gal with hardener) for the barrier coat, and keep my West System for projects on the boat.

Unfortunately, the local stuff seems to behave quite strangely when applied to surfaces like a paint. It acts a bit like water on a waxy surface, despite the gelcoat being sanded, cleaned with acetone, dried, and wiped down.

I've tried a number of different things, including changing the surface prep, ensuring the mixed ratio was correct, and thinning the resin by 10% with denatured alcohol. None of these actions produced a result that was much better.

Meanwhile, test of West System produced a satisfactory result on the first try.

The two photos below illustrate the results of the two resins on the hull -- both have a small amount of graphite added. The first is the local stuff, and the second is West System.

We're pretty certain that we're going to use the West System at this point, as I just can't trust the other stuff, especially after all the hard work we did to get the boat down to gelcoat. Unfortunately, West System is $300/gal (including hardener) here on-island, so we'll only put on as much as the one gallon we have will give us.

[rbk]

Originally Posted by Krogensailor

I think we're getting epoxy paint/primer and epoxy mixed together here. Chemically bondeing epoxy primer to top coat makes good sense. But epoxy barrior coat, as in west epoxy rolled onto the bare hull, needs to kick off completly before any painting is done. And the thought that there is a , " risk enclosing the volatiles and preventing the epoxy from ever curing properly", wouldn't be the issue. Epoxy, as such does not cure by off gassing of volatiles. It is a chemical linking reaction, not an evaporation one. So I'd say you apply your epoxy barrior coat's. "Put on the epoxy and when it has dried enough to leave a fingerprint in it without getting any on you finger, it is ready to overcoat". Yes, overcoat with more epoxy, not paint. It has to cure completly before priming with any paint. Let it kick some number of days. Sand with, say 120. Apply primer and then do the fingerprint, or what have you test, apply bottom paint. I'm thinking that would be the correct sequence. I could be wrong.

X2 there are many documented cases where antifoul was applied to barrier coat before it was cured and requires complete and messy removal.