Gelcoat patch yellows around the edge?

I've got gelcoat patches on my boat that have yellowed around the edge of the patch. What is the cause of the yellowing?

 

There are a few different reasons for patches turning yellow. It's hard to say without looking at them, but here's a few.

1. Over or under catalyzed gel coat.

2. Styrene or other solvents used to reduce it.

3. Large amounts of patchaid type products.

4. Poor quality, or old gel coat.

5. Thicker than normal gel coat (30+ mils not likely in this case)

6. DMA added to gel coat.

7. Gel coat not mixed well when Catalyzed.

8. Gel coat not mixed well before being taken from the container it came in. This may happen when it was purchased and repackeged by the place you got it from.

9. Poorly cured gel coat. (sprayed too thin or in too cool conditions)

10. Too much heat from the buffer.

 

The only place it turns yellow is on the blend line with the original gelcoat. The gelcoat patches were very poorly, nonprofessionally done, and I suspect they were not sprayed, but smeared on thickly and then sanded. Based on my suspicions several of the things you mentioned could have caused the problems. Thanks for your input.