Gel coat color matching & pigments question

Hey guys was hoping to gain some insight.
When I do Gel coat repairs on an slightly offwhite colored boat I add yellow oxide and maybe a touch of black to tint the white Gel coat to match. I am having a problem. The color appears to match but once it has kicked of and I sand and polish it out, the repaired area has a green hue to it. I thought perhaps I was over catalyzing (I try to always stay at 2% MEKP) but it just happened again the other day and I am sure the ratio was good.

Could it be that the base white Gel coat has some blue in it and the blue and yellow are making green.

I would really appreciate some feed back

Thank you

 

The basic white pigment generally is not a problem, it's almost always titanium oxide. Bright basic yellows are generally pure. But the black is maybe the source of the hue; some blacks are not pure but a mix of colors to give it a hue (black-black is very flat), maybe "your" black contains blue as the nicest blacks are in reality very dark blues... Try also less peroxide at 1.5 %.
Try a mix of white and yellow without black and observe. If there is not hue, it's the black the guilty.
The same problem happens when trying to make a "nice" white with paints.

 

Thanks for the input Voyager

 

You're welcome. I hope to have been of some help and that you'll resolved the problem.

 

I have also had the green hue problem with out using Black and only using the Yellow oxide (or Raw sienna) I think is the correct name.
The pigment I most often use looks like a goldish brown yellow.
From what I have read researching color matching, and in my own experience
this Dijon mustard color is used quite often. That makes me think it's not the actual pigment that is the problem

It isn't every time that they turn green so I'm kinda stumped

 

Catalysts change greenish colour ratio is part of the problem but type is the major concern. gel coats usually have there own type of catalyst
Also styrene !! if you using styrene to thin with changes colours
!! I always use clean acetone straight from the container

 

Some resins can give greenish hues by themselves.
I tried always to use as much much as possible metal oxides as there are the most stable than synthetic colors.
Talc as filler gives also a green-grey hue very difficult to hide by.

It's true that styrene changes the hue. I've also remarked that isophtalic resins have less hue (apart the fact that orthophtalic are generally not suitable for marine use).
The only thing is to make trials and maybe a little prayer.
It's very difficult to match colors.

 

Thanks Guys !!

Tunnels I will check with my supplier I was not aware that Gel coat uses different catalyst.

 

Most every country i worked the gell coat uses a differant catalyst to what the resin uses
Its a differant reaction and dosent produce as much gasing off so cuts down on Porosity with in the gell coat its self !!

 

Tunnels

I went to the supplier today and they said they don't offer a different Catalyst
it is High point 90 Mekp

Got any suggestions...Thanks

 

The greenish colour is usuall the catalyst reaction with the promoter in the gel coat . apart from that cant help !! I Have used pearl from a car painter to help blend colours !! pearl make light refraction and discises the light a little !!

 

as mentioned it is the catalyst/accelerator in the gel that causes the green colour.

whites are a pig to match even without this problem. (just check out how many different shades of white paper is for example. You eye is super sensitive to small changes, and the green hue your eye is very sensitive to)

You won't always get the same colour shift on curing as it will depend on the thickness, mix ratio, humidity, temperature...in fact everything.

As also mentioned some shades of white you won't be able to match perhaps if your base gel resin starts too grey/green. Adding tinters will just make it dirtier/darker.

And in the fact that the original gelcoat will have yellowed over time and you are left with a hard job.

The solution? Luck, skill, practice, patience. It is an art.

I work with new build yachts and the parts often come out different colours and this is with the same batches of resin.

A repair is a repair. Of course it would be nice to get it faultless. Whether your customer wants to pay for 5 attempts is another matter. I am painter by trade. I always tell the paint repair customers that I will not guarantee a faultless repair. It is just storing up trouble to promise something that actually you have insufficient control over.

 

About 20 years ago i did a repair on the outside rear cockpit coaming of a Leisure 23 yacht in our Club. It was a deck grey moulding & the hole was about 3" round, i had two goes at matching the gel, I though it was spot on but guess what it went greenish.
Every time i walk past that boat it grins back at me! Glad im not the only one!

 

I'm glad to know it's not just me to

I would not have thought it was a reaction between the the catalyst and the promoter. I was sure I was doing something wrong with the pigments or my base white gelcoat was not a clean enough white.

Thanks for sharing that your new parts can turn out a different color even with the same gelcoat, that illustrates the point that ratio, temp, and humidity can change the color.

Cheers

 

Try using a clear gelcoat and use a white pigment to make it white.it could allow you to do a fade into the old gelcoat