Decal Shadowing

When the decals are applied, they use what is known as an application fluid, such as Sure Glide and some other brands. This fluid acts as an activator for the vinyl decal adhesive and also allows the decal to be positioned in the fluid until it is squeegeed out and all bubbles are removed. I am sure that the discoloration is caused by the adhesive and application fluid reacting with the gel coat. It may take a more aggressive rubbing compound to remove all of the discoloration. Try it first in a small area that is not too noticeable and see if rubbing compound is necessary. Do it by hand and keep checking the results to see if this takes care of the shadows without going too deep into the gel coated surface. After the rubbing compound, polishing compound can bring the shine back.