Gelcoat story

[DanielR]

Since I've never done enough gelcoat repairs to be able to answer my own questions. I'd appreciate anyone knowledgeable to chime in.

I was shooting unwaxed gelcoat (fire engine red pigment) with 5% styrene over existing theme white/burgundy 19 footer.
After using 1gal shooting two coats I've realized my catalyst ratio was way off. As calculated Ive mixed only .35% catalyst. Recommeded for that gel is .75-1.5%.
Weather was in 80s.
As soon As i found out catalyst was at low percentage i stopped.
That was last night.
Today,before i left to work i quickly peeked in to see the gel, touched it and left no fingerprint. Once i get back home ill do closer investigation.
Forecast for today is sunny in mid 80s. In tent that im spraying is probably 20degrees more easy.

Now.
Is there any hope for me to continue shooting remaining layers? How do i make sure gelcoat cured enough to lay extra coats on top? wipe it with acetone see if it comes off?

 

[DanielR]

Re: Gelcoat story

Followed up on the gel today.
20hrs after painting and gelcoat is hard but tacky (unwaxed coat).
I used razor blade to scratch the coat at random spots, no sign of it being uncured.

I plan to shoot remaining amount, this time with 1% of catalyst.

pic of 1 coat down as of last night:

 

[sswheeler]

Re: Gelcoat story

I might be way off but, I think that by having a reduced catalyst, the gelcoat would have a longer cure time. It should still cure fine. It sounds like you have a good warm spot for it to dry. I would go for it.
Good luck.

 

[RIDEPATE]

Re: Gelcoat story

Looks good Dan. I'm with ss, it will take longer to kick, but should be ok.

 

[ondarvr]

Re: Gelcoat story

At .35% it may get hard, but will most likely never cure completely, this means it won't be as water and UV resistent as it should be. Going over it with a Gel coat at a higher % will help in some ways, but it's not a 100% fix.

Recommended catalyst ratios are typically from 1.25% to 3%, with 2% being close to ideal.

Will it hold up......for a period of time yes.....will this be as long as you want it to.....no way to tell.

 

[Yacht Dr.]

Re: Gelcoat story

Followed up on the gel today.
20hrs after painting and gelcoat is hard but tacky (unwaxed coat).
I used razor blade to scratch the coat at random spots, no sign of it being uncured.

I plan to shoot remaining amount, this time with 1% of catalyst.

pic of 1 coat down as of last night:

You should be fine ..

As long as you have hard cured gel on the outside when its time to sand/buff and does not Peel/melt off the blend lines .. you should be fine. You will Know if the gel is not cured enough to blend out ( use up to 2k wet for final sanding to reduce the amount of Head needed to Melt your gel for full buff out .... .. ).

YD.

PS. I have only found ONE time in gel apps that the gel Failed .. and that was because the application was too cold and very under-catalized ..( failed gel application not done by me lol )..

 

[ondarvr]

Re: Gelcoat story

Buffing and gloss aren't the issues with an under cured base coat, it's how well it holds up to long term water exposure. The under catalyzed layer is what's being relied upon for the bond, it needs to be cured well enough to withstand breaking down due to the effects of water. For the most part he will be sanding and buffing on the layer with more catalyst, which should work fine.

When you see boats with extensive crazing over or under catalyzation is the main culprit due to poor crosslinking

 

[DanielR]

Re: Gelcoat story

I attempted to shoot gel today, early morning approx 70 outside, in the shade (trees blocking the sun).
Mixed 2% catalyst.
3/4 in the way of spraying one side of the boat, webbing like consistency started coming out of the gun. I knew something was wrong. Opened the pot, gel was already beyond jello.
I put the hand into the pot to grab, yes grab already gelled coat. It looked as if i was ripping someones heart out since gel was red and I was holding big chunk of it in my hand.
Rushed to flush it with acetone to save it. Took some time to push all the chunky gel thorough hose/tube but at the end i managed to clean it.

Lesson,
Follow manufactures recommendation.
Different gelcoats require different catalyst ratio.
Black colored gel (Cook Composites) that i sprayed took 2% in a hotter day and didnt have single problem with it.
Red one (Noahs sprayable) gel kicked in under 5min at 2% ratio. Recommended is .75-1.5%. Pressure in the pot made the gel harden even faster than it normally would do.

 

[ondarvr]

Re: Gelcoat story

Sorry for what happened.

Noah doesn't make gel coat, they buy it from someone else, why they would use a formula that cures so rapidly is a mystery, but many retail places do the same thing. While the gel time is fast, the chemistry still relies on the same chemical reactions to crosslink and cure, so the amount of catalyst is still important even though the product is too fast to use in the real world. When we supply gel coat to customers that are going to use it in a pressure pot, or in a manual type application where gel times need to be extended, we set the gel time in the 20+ minute range. You found that out with the black you used before.

The other thing that wasn't mentioned earlier is that when the conditions don't allow you to mix a full pot due to premature gelling, you need to mix smaller amounts each time, but still use the correct amount of catalyst. This keeps the catalyst ratio in the correct range so the physical properties of the gel coat are where they should be for a long lasting finish.

We make gel coats with gel times from 3 minutes to around 35 minutes and any retail outlet can ordrer gel coat with longer gel times so the product is much easier to use, why they don't do it seems crazy. This is the most common complaint with both resin and gel coat sold in the retail market.

 

[DanielR]

Re: Gelcoat story

Originally as PMs, i decided to keep this information for anyone who might be spraying with custom darker shades of gel.

This is a follow up on a boat that i have sprayed with custom red gelcoat.
After two days of drying i washed the PVA with water and i instantly noticed gelcoat getting bleached due to contact with water.
Color of the gel became red falling into pink shade.

Used was noahs neutral gel for spraying and spectrum pigment .Mix was done by me (4oz pigment to 1 gal of gel).
Once ive noticed the water is washing color of the gel, I let the hull dry and i applied wax on top to seal it (thinking it might not dried enough before i stripped pva).
That was two days ago.
Today, to clean the wax off (it was mold release wax) i used por15 marine clean. As i was brushing, washout of rinsed water was pinkish, which i think was the leftover uncured gel that got scrubbed away.
Color of the gelcoat after rinsing was lighter than the color pigments that i originally mixed. After drying the gel darkened a bit but it was still rather light.

I will continue sanding it which i found grit 220 works best to knock initial roughness. But Im curious why the gel came out lighter in color after i sprayed it comparing to what was leftover cured gel in the bucket.

Original pigments with gel (uncured i left as a sample):

Cured gel on the spoon (no wax added), you can see the line where gel was exposed to water - ligher shade

Hard to tell since camera does its own color balance, but the surface is on the pinkish side:

You can see it clearer on this photo, except the area which i have sanded. I tested to see if a buffed out gel would darken - which it did but i wasnt the color of the cured unsprayed gel.

 

[BWT]

Re: Gelcoat story

gelcoat is a finicky creature; catalyst levels, humidity, temperature all play a factor in how a wet color cures. Higher catalyzed levels, and it darkens; lower and it lightens,(granted it's a subtle difference, but noticeable as a patch). I tend to mix my gel (no Durtatec) at 1-1.25% and color stays pretty consistent. (1% on hot days, and 1.25 or so on cooler days); 2% for top coats with Duratec.

 

[ondarvr]

Re: Gelcoat story

The blushing is common when water comes in contact with gel coat that isn't fully cured, or dark colors that spend a good deal of time under water. It normally doesn't turn white quickly if PVA is used and it's cured properly, this is where the higher catalyst ratios come into play.

There can be other issues with custom blends too. One is when pigment is added it tends to inhibit the cure, so with colors that need high pigment loading the surface cure can be poor, reds normally need a lot of pigment to hide well. 4 oz per gallon seems on the low side for a red though. When colored gel coat is purchased from the manufacturer it is QC'd after all the pigments have been added, so the gel time and viscosity are adjusted back to where they should be. If the pigment loading is low, then the hide may be poor, which means if the gel starts to blush it will be noticed more easily.

Polyesters tend to cure poorly in a thin film, they don't generate enough heat and energy to crosslink well. Gel coat normally has a laminate over it which can help drive the cure, without that help it doesn't always cure tack free. Low emission gel has an even more difficult time curing on the backside even with wax or PVA. Patchaid type products are formulated help the thin film of gel coat cure better which results in better gloss and color retention.

 

[Yacht Dr.]

Re: Gelcoat story

Hmm.. I think we should backup and regroup ..

Taking it from the top.

1. you sprayed your first batch with .35% cat with no wax. waited a few days to cure.

2. then you tried to spray it again with 2% cat ( wax ? or reducers ? ) and it started kicking off in the gun.
Did you end up spraying the rest with 1.5% after flushing your gun ?

3. after a few days you rinsed the pva off..then applied mold wax for a few days. ( did you wipe with acetone before you waxed ? ).

After you buff a spot..does the water seem to bleach it out ?

Have you tried Acetone wipe just before buffing it.. ? Or will acetone attack the gel still .. ?

I have had this happen to me one time spraying blue. While wetsanding I could see the "bleaching" affect.
After buffing it out and a few weeks later it became less and less noticeable.

I attributed this due to undercatted gel ( I was in HOT weather and cut back on my % ).

I would try to acetone wipe and buff a spot ( do not wax it and do not use compounds with silicates or petro additives .. 3m Imperial works well ).

Get back to us .

YD.