This project was easy... I needed to swap the decals from a boat 13 years old, which means years of sun exposure to the gel coat will contrast with the virgin gel coat beneath the decals. If you replace decals with the exact same one, you'll be fine. But for an updated look, in this case stainless emblems with a different size, you have a problem if you don't restore the gel coat properly.
A couple things... This project on the 2003 was not extreme, because the gel coat looked new, until we pulled the decals. (I did this once on a 80s Sunsetter, red, and it was much harder because there were more years of exposure that created a chalky red.)
A quick polish will make the contrast go away briefly, but trust me it will not last. It's best to clean, wet sand and then polish. This project was for decal swap, but the same steps work for restoring color regardless.
In this case the gelcoat was in great shape, because it gets polished and waxed regularly. So it's really not the best example for demonstration.
These pictures are not very good, but you can see not only the outline but the gelcoat has a different gray then the grey thats been exposed to the sun. Polishing is just a temporary solution, but you don't want to put new decals on until you make a permanent adjustment.
1) So I first washed the boat so there was no dirt in the area I was working on. I used acetone to remove the decal residue completely.
2) Before starting the wet sanding process, tape off a section off the area you are working on. You have to be patient with this process, when you wet sand from lower to higher grit paper, you cannot skip a step and its easy to get lost even on a small section. Tape helps you keep track.
3) Only use a sanding block, do not use a rotary or try to wetland with your hand. It's also best to get good wetsand paper, and let it soak in a bucket first. For this project I started with 600 grit, the 800, then 1000, then 1200, then 1500, then 2000. Had to clean in between every step, and keep the sanding block/paper wet the entire time. Also, it helps to move the tape out with each step. I found a little car wash soap in my water-dunking bucket was helpful because it made it a little slick as well.
4) When it was clean and dry, I taped off the next section and repeated the process.
5) At the end, I removed all tape and polished the entire area with a rotary buffer using a wool cut pad and Maguiers 105 (heavy cut) polish. Although not needed, I then went to 3M's Finesse it polish for a final finish.
I almost waxed it before realizing I should put the new decals on first so they stick better. Not sure if I needed it, but I used a little more acetone to clean the gelcoat again before the decals. Once they were on, I added the wax to the whole thing.
It's best to cover your trailer too, I did not and it got pretty mucky from the wet sanding you can see in the picture. It does wash right off though.
A few boat trips and sun exposure, and the gel coat contrast is still completely gone. Permanently fixed, until I decide to change decals years down the road.
I'm a hack, so I'm sure professional detailers can jump in here and improve the process. But this works very well.
EDIT: I found pictures of a previous project I had to do to the 87 Sunsetter. It too received new decals, but the gel was in much worse shape to start with and was a better example of before and after. Check it out... I hope it's helpful for anyone who shies away from an older boat because it was chalky!