Polyester Flowcoat stick to Epoxy?

[minaret]

Originally Posted by colemj

Awlgrip 545 is a 2-part epoxy primer. It is the same basic chemistry as your current epoxy build, so I don't understand why Minaret thinks that epoxy is OK for use under polyester gelcoat but your current epoxy is not.

The primer will be a mechanical bond and you will need to sand it before gelcoat. Since you have old epoxy that will be difficult to really clean well, and you need to do some filling anyway, an epoxy primer like this is a good idea to give the gelcoat a new clean keyed surface to adhere to.

If you read the post threads again, you will find only two people saying you cannot put gelcoat over epoxy - only one of which seems to actually have experience doing so. The rest of us have related direct experiences of having no issues. Including an epoxy manufacturer.

But if you are still concerned, then nimblemotor has your answer - just use tinted epoxy. Epoxy does not tint as well as gelcoat, but for use in an anchor locker that should not be an issue. I wouldn't use the tabletop stuff he recommends, though.

Your other option is to just fair it out with a couple passes of slightly thickened epoxy and paint it.

Mark

Yeesh. 545 is a "modified epoxy". And, in fact, there are many different types of "epoxy". It is a catch all term anymore. I can assure you 545 is an industry standard tie coat. Ever seen it blush? No, you haven't, because it has no amines in it. Which is what causes the compatibility problem with poly on epoxy. WEST's testing meshes perfectly with mine, they just don't take it far enough and as usual are misleading in their favor. I have done gel on epoxy a number of times, and it always seems fine when you do it and will pass an adhesion test with flying colors. It's not till it gets a year or two of exposure that failure occurs. I have discussed this at length here before. For me, the case which clinched it was a fleet if Star class Olympic race boats which we finished in poly on epoxy at the insistence of the fleets manager, despite my warnings. These boats had epoxy laminates done by the manager, by two different people in our shop (including me), and some older stuff of unknown progeny as well. Within two years, failure had begun in all surfaces, despite several different products used by several different people. We removed all coatings and tie coated with 545 before gel coating. Those boats still look great today. The fact is, true epoxy keeps leeching very small amounts of amine long after it's cured. This amine destroys the bond between poly and epoxy over time. Just because adhesion is good out the gate means nothing.

[minaret]

Originally Posted by donradcliffe

If you are doing repairs on the exterior remember that, epoxy, however tinted, is going to turn brown in the sun.

+1. And, it has no UV resistance and will chalk and fail over time when exposed. That primer has a lot of functions!

[colemj]

There are many different types of amines also. "Amine" is a catch all term describing only a particular chemical class of compounds.

The amine content is in the reactor part of the epoxy chemistry. Some, like West System 206, use mostly linear and multifunctional amines that cure faster and harder than other systems using monofunctional and cyclic amines, which do not blush.

These blushing amines are not part and parcel of all epoxies. You give 545 as a good example of one where they are not. "Modified epoxy" is a pretty meaningless term, and I assure you that 545 has amines as part of its chemistry. In fact, a quick glance at its MSDS shows it contains 1% by weight triethylenetetramine (oddly, a linear multifunctional amine).

West Systems 206 is the worse epoxy for free amines and their subsequent blushing. Other laminating epoxies do not blush or leach amines at all. West Systems 207 is pretty clean, and Raka and MAS make very good non-blushing epoxies.

It is a good bet that even if the OP's boat was built with West System epoxy, the amine blush and leaching is long gone by now.

BTW, note that I did say your 545 recommendation was a good one. I was just pointing out that it is OK to put poly over epoxy - and that the "common wisdom" that this is always bad is wrong. The 545 type of epoxy proves that point.

Mark

[minaret]

Sure, what you are saying is technically true. What I am saying is that in my experience, in practice, in the real world, I have seen multiple failures with multiple resin systems. Why gamble when a tie coat is quick and easy? It's just not professional, and I certainly wouldn't even consider putting our standard ten year warranty on it. It's too much work getting a nice long lasting finish, why balk at adding one more step to ensure longevity?


By the way, I refuse to use MAS because their "non-blushing" resin blushes worse than any other resin system I've ever used. Several other "non-blushing" systems have given similar, if not as extreme results, particularly in less than ideal conditions. At least WEST is predictable and reliable across a wide range of conditions.

[ggray]

I have used an epoxy product I got from Progressive Epoxy that should do the trick.
It's been a few years, but IIRC it has kevlar fibers in it. Light grey in color, it is very thick and results in a smooth, hard surface.
I don't have any connection to this company.

[Marqus]

A big thank-you gentlemen for your posts on this topic.

Through your expert contributions many readers and myself have gained valuable knowledge on fibreglass usage that's not disclosed in the fine print on the packet.

Given the versatility of the tie-coat, my plan is to find and buy some before applying my flowcoat to the anchor locker floor.

I'm also planning a little experiment on the side with a small amount of epoxy resin, a bit of filler and pigment to see what sort of epoxy paint I can mix. I don't have epoxy-specific pigment, but I do assume that polyester (gelcoat) pigment won't harm the epoxy.