Vinylester reacting with ISO/NPG gelcoat

I’m encountering a few issues and would appreciate any guidance.

After applying gelcoat in a mold and letting it cure until mildly tacky (about 4 hours), the vinylester resin appears to dissolve or disturb the gelcoat more than expected. I understand that vinylester chemically bonds with gelcoat, but the interaction seems too aggressive and is causing alligatoring in some areas. Any suggestions on how to prevent this?

The two videos show how the gelcoat dissolves and the alligatoring. The first gelcoat layer was tinted blue and the second was just white - this is why you see blue shade under the alligatoring portions.

Should I use one layer of polyester resin over gelcoat (less reaction) and then move to vinylester (prefer to use vinylester because of it's superior physical properties)?

materials -

• Fiberglass supply depot Vinylester resin
• Total Boat Gelcoat without wax ISO/NPG premium gelcoat.

 

Polyester or VE makes no difference.

The only reason for alligators is under cured gel coat, there are many reasons that it may be under cured though.

 

How long have you been storing the gelcoat and catalyst?It may have deteriorated.I don't know anything about the climate in Alabama at this time of year but in a normal workshop environment I would expect properly mixed gelcoat to be sufficiently cured inside four hours.

 

Both you and @wet feet were right — the gelcoat simply wasn’t cured enough. After waiting another day, I was able to apply the vinylester without any issues.

What still confuses me is the general guidance. I initially waited about 4 hours in a temperature-controlled room (18°C / 64°F). As shown in the video, the surface was tacky but did not leave a fingerprint, which matches what I’ve read as the indicator for applying the next coat.

 

The cure time can be very specific to the exact gel coat brand and formula, plus the color, they are all different, and any directions are generic.

Plus 64F is at the low end of where you'll get a reliable cure, so actual lamination times will be unreliable.

 

Tacky dry is the term used. In general, the next layer is applied when it is tacky dry. Color does not transfer to the fingers.

Gelcoat that has been left too long to dry/cure will be attacked by the resin of the next layer and will cause blister/bubble/delamination.

 

Gel coat that has sat too long does not create a problem right away, all will go well.

The potential problem is a poor bond to the gel coat that doesn't show up until later, which doesn't happen frequently.

 

Time interval between Gelcoat & First laminate https://www.boatdesign.net/threads/time-interval-between-gelcoat-first-laminate.60330/

 

This is not what you indicated in your statement. You indicated that he may have waited too long to apply resin, which may have caused the problem. The problem he experienced is never from waiting too long.

I have 60 years in the industry, 30 of that working directly for the largest global resin and gel coat manufacturers in the industry doing tech service, product development and training.

Gel coat time frames for laminating are recommended for "as soon as possible" for a few reasons. There are thousands of different formulations of gel coat, each one will have a different time frame, and the time frame is very dependent on ambient conditions. So the safe answer is do it ASAP after its ready. We made gel coats with a four minute gel time that were tack free at eight minutes and laminated on in twelve minutes. And we made products with a thirty minute gel time that were still rather gummy on the backside three days later.

Prerelease is a potential problem of waiting too long, this can be very dependent on the exact gel coat, ambient conditions and gel coat thickness.

Another consideration, and is the primary concern, is contamination of the gel coat. There are frequently other processes of production taking place in the same facility, so grinding dust, saw dust, PVA, other mold release products, dirt, etc, may land on the surface, so the less time between when it's ready, and actual lamination the better.

Now let's look at the real world of marine manufacturing. For high production and low production of small boats it's not hard to laminate on the gel coat within a few hours. Now let's scale that up. A 100' craft that is being infused may take several hours to gel coat, then you need to wait long enough so you can load the fabric, cores, etc, then place the bag and lines, test it, then infuse, this can take days to complete before any resin hits the gel coat. And it works.

If you plan to skin that hull first, it still takes longer than the officially recommended time line. And that's for a 100' craft, now let's scale that up to a 200' craft, and yes I work on these projects.

Alligators are only from under cured gel coat, not overly cured gel coat. Any problems from laminating on overly cured gel coat show up significantly later, as in maybe years sometimes. And that may be peeling, flaking, blisters etc, these don't show up minutes after you start laminating.

And yes, bond testing has been done at various times from very short to a week or more, the bond is actually quite good for much longer than the official "cover your ***" reccomended time. But it can vary significantly from one formula to the next.

 

We are also a member of ACMA and we follow the best practice in the industry in accordance with the manual.

We frown upon overnight curing as we have to sand the surface to promote adhesion.

While I do not discount your experience as most of what you are saying is true, we do manufacture boats (under Lloyd's supervision/Rules) and granite/synthetic stones in the yard. Just different approach.

We have an environmentally controlled laminating area, cutting and assembly is in in a different enclosed area. We use gelcoat spray gun. Finish in 20 to 30 mins.

 

Much of the ACMA best practices information came straight from the testing and recommendations from our applications manual. Almost to the point of our page numbers still being in the copy and paste. That's why I know how it was determined and how they decided to word it.

We have people on the boards of these organizations. And the center stage of the composites demonstrations at ACMA is and was put on by our group.

And back to the original topic, the problem the OP experienced was from under cured gel coat, not from waiting too long.

So you finish spraying a 200' hull in 20 to 30 minutes and have it skinned in a few hours, interesting.

Prerelease is the biggest risk in waiting overnight to laminate, and it's a very common method throughout the industry.

 

My stick is bigger than yours conondrum.

 

That's not the intent, the intent is accurate answers to the questions.

You wanted to "one up" me by linking irrelevant stuff, then ACMA best practices. I responded with factual information about both subjects. And the correct answer to his question.

 

So you feel I am trying to one up you. So sorry to rain on your parade. Don't be so sensitive. While I admire your knowledge on materials, your ramblings on post #9 shows you don't have a grasp on how shipyards are setup and how it operates. They do exist in "real world" not only in imagination. We did practice ACMA and my manual is still intact (somewhere). Don't stretch yourself too much.

 

I have supplied gel coat and resin to quite a few builders of yachts that range up to 200', even in Asia, and participated in the gel coating while training them. Their build methods varied a great deal, some skinned the hulls then infused, some didn't, others used impregnators, some used clear gel coat and one builder never used gel coat on the hull. I was also the Marine market manager while at one gel coat company, that was after I did tech service for the major boat manufacturers in North America. I am very familiar with the industry.

Your replies are typically on target, at first I thought you misunderstand what the OP asked, but then you doubled down on it.

Here's a link to a local yatch builder that I've worked with for decades. I worked with many others in this region too.

https://westportyachts.com/