The first step for preparing boat surfaces for the application of vinyl is cleaning.
IPA (isopropyl alcohol) is generally recommended unless the substrate manufacturer states that there is an incompatibility. Caution: many cleaning agents can leave a residue that either prevents vinyl adhesion or attacks the vinyl adhesive, such as Windex® and other glass or surface cleaners, Rapid Prep®, and degreasers.
Surface temperatures should be between 50 and 90 degrees Fahrenheit before attempting the installation. As a general rule, if the surface is hot to the touch it’s over 90 degrees Fahrenheit. Adhesive becomes brittle when it gets too cold and gummy when it gets too hot. Applications outside the recommended range of 50 to 90 degrees Fahrenheit reduce the adhesives’ ability to perform.
Make sure you SUPER clean the areas you are going to be applying to. On fiberglass, a firm alcohol cleaning with a clean rag to remove any wax/lime/salt is all you’ll need.
Wipe it several times just to make sure it is very clean.
Optional, from others that have applied graphics to boats: Make sure the graphics are all above the waterline while the boat is at rest fully loaded in the water & use a prep like 3M Primer 94 to prepare the surface for the vinyl (apply a very thin film of it and allow 15 minutes to tack up). Follow up with an edge sealer like 3M Edge Sealer 3950. All you want to do is make sure water does not get under the leading edge of the vinyl and cause it to lose adhesion.