Color-match not matched after hardened

[scherzoja]

I tried some gelcoat color matching, using the spit-on-celophane method and found some interesting results.

The test piece is a piece of interior liner that I removed. I sanded and polished it back to shiny, then proceeded to color-match.
After about 2 hours of adding drops of yellow, black, and brown to white gelcoat, I got the color to match under the celophane, but a weird thing happened when I catalyzed, applied, then sanded a test spot.

The patched spot is darker than the uncatalyzed resin. The stranger part is that the dark spot is slightly smaller than the original test hole. I used a burr to grind out a spot about 1/4" x 3/8", then put some catalyzed gelcoat in tha test hole. The finished test shows a dark spot of about the same shape, but slightly smaller.

Last night, I noticed the same thing with a test 'scratch' that I did with a screw driver. After hardened/sand/polish, the patched scratch is visible (darker), but only a small portion.

This darker shade does not exist in the uncatalyzed resin

It seems like the thinner edges blended in with the original, but the deeper center got darker.

I don't recall seeing this difference when I applied the catalyzed resin to the test hole.

What's going on?

Thanks

[SailsWithFists]

You need to mix your color 2-3 shades lighter than the test panel to get it to match. Paint/gelcoat always dries darker.

[skipmac]

Originally Posted by SailsWithFists

You need to mix your color 2-3 shades lighter than the test panel to get it to match. Paint/gelcoat always dries darker.

Any other hints on patching a small bit of gel coat? I'm about to patch a longish crack on a rub rail which will be my first go at gel coat repair.

[SailsWithFists]

I only used red, green, blue, yellow, and black, as tints to color match. Once you think you have it right (2-3 shades lighter), put a drop on the surface, brush it out a bit, then let it dry (or force dry it with a hair drier) to check the actual match when dried.


You will need to remember/document your mix ratio in order to get it right again later. It was not uncommon to spend 30 minutes to an hour getting a color match manually, with multiple test samples. I did it professionally, so it might take you longer.

[Suijin]

Originally Posted by skipmac

Any other hints on patching a small bit of gel coat? I'm about to patch a longish crack on a rub rail which will be my first go at gel coat repair.

This is an excellent primer on color matching gelcoat. Short story- it's time consuming but not that difficult.

https://www.youtube.com/watch?v=yoURRmkD8MY

[scherzoja]

Thanks for the insight on the color changes when dried. I knew a similar phenomenon occurred with paint and didn't think it occurred with gelcoat. I'll experiment more tonight.

That video is interesting in that he did not mention color changes when dried.

[puffcard]

Thinning Gel coat with acetone, also causes it to darken.