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Old 10-25-2016, 04:45 PM   #1

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I emailed North American Composites about a yellowing issue with some marine polyester products. The NEXt day a sales rep and a chemist from Interplastics Corp came to my door to answer a few questions. How is that for good customer service! Best ever! I am very appreciative to these guys.

So my clear marine gel coat was yellowing prematurely on some parts that were made just a couple months prior. Here is what they said:

Me - I am thinning my gel coat with acetone at 12%.
Them - Thats not good. Acetone absorbs moisture, that is bad for resin (gel coat) as it will not 100% cure due to moisture. And chemically you are changing the properties that make the gel coat have all the desired properties. Thin with MEK. (not the same thing as mekp guys). Try 3%.
Me- I have spoke to other guys who spray gel coat. Some say they thin with acetone, some thin with mek, and others Styrene monomer.
Them - No don't thin with styrene. Two things happen. Too much styrene wont crosslink with the resinand you will have extra styrene that is unlinked and causes yellowing. You will know if your gel coat is styrene rich if the surface yellows and you are able to buff it out.
Me- wow I didnt know this.
Them- Catalyze at 1~2% with mekp.
Me- Is this catalyst the right stuff ( Cadox L-50A?
Them- Yes that is a great catylist for all polyester and vinylester resins and gel coat.
Me- I noticed Silmar 249ah kicks too fast catylized at 2% it is gelling in the cup. I use 1% with Silmar.
Them- That is good it will cure at 1%.
Me. I use acetone at 2% in Silmar249a as an air release agent.
Them- Again it is not good to use acetone in resin. We suggest BYK-555 air release agent.
Me - Ok I have used BYK9920 and it works great.
Them- We sell byk555 in pails.
Me- I want a sample of Silmar acrylic modified clear resin.
Them- No its too good and you will want to buy it and we don,t sell enough to stock it. Sylmar 249ah sells well for us.
Me- thinking how bad I want to try the acrylic modified Silmar!!
Them- Back to thinning gel coat you want to use as little mek as possible to thin, always use the least amount to get it to spray.
Me- I,m thinking in my head without a 2.0 tip on my hvlp gun no gel coat is going to spray through a 1.4 mm tip without being thinned 10%. Looks like I need a 2.0.

Anyways I just wanted to share this information with you guys, I hope it helps others as they helped me.
Also if you are curious, Silmar249ah has uv absorbers in it to help delay yellowing and such effects from uv light.
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Old 10-25-2016, 09:55 PM   #2

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I've used the 249 stuff before and it kicks fast. Basically I'm just agreeing with you haha
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Old 10-31-2016, 08:33 AM   #3

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Thanks for the insight!
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Old 11-08-2016, 03:09 AM   #4

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2 more thoughts:

You can ask for a slower catalyst

You can add acetone up to some 5% to the gelcoat, but spray from a distance (2-3 ft) so the acetone can evaporate before it hits the mold, and have a somewhat slow gelcoat, so the remainder of the acetone can evaporate before the gelcoat sets. On the other hand, a gelcoat should show a decent amount of cure to prevent further problems (yellowing and elephant skinning)

But this guy is right, undercure and residual styrene will cause yellowing.
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Old 11-09-2016, 08:11 AM   #5

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Are you using the Silmar as a gel coat? Is this "gel coat" being sprayed into a mold or are you post applying to a molded part?
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