I have to say that before you do anything you should determine why this crack is there in the first place. If you skip this step you may find yourself right back where you are now in months or years to come. Why do the work twice? After you have an idea what has caused the crack you can take the appropriate steps to make a lasting repair. Most cracks, to mention a few, are from some sort of stress but some are caused by water infiltration and freezing. If the area flexes under pressure reinforcement may be needed in the form of fiberglass. Speaking of fiberglass that should probably be the final step of repair because it will add strength to the area and create a surface that can be faired, primed, and painted. Keep in mind that if you plan on using Gelcoat to finalize your repair it will not stick to epoxy well if at all, but that is another subject. That said the entire area should be ground back to remove every trace of discoloration, chips, and hairline cracks. After grinding, if find you have removed more than one-eighth of an inch of material from the area you must use fiberglass to make the repair sound, as an epoxy filler may just snap off. The surface also needs to be free of moisture solvents and oils including wax. A good way to check for moisture content is to seal the area with clear plastic and leave it that way for a couple of days. If moisture is formed under the plastic then the area needs to be dried out more and checked with the plastic again. You can also get a moisture meter that will give you an instant reading of the moisture content. The trick to keeping your epoxy from sagging is to thicken it to a fairly stiff mix and only apply it in thin coats. While this may take several layers to complete the repair the end result will be a better repair. Be sure that you adhere to the epoxy instructions, especially the time allowed before you need to sand the surface between layers.
As for the interior, if the surface is smooth it should be sanded to the grit the paint manufacturer suggests and wiped down with the manufactures suggested solvent. If it is not smooth sanding is still required in most cases followed by scrubbing out any grime from textured areas with the suggested prep solvent and possibly some sort of brush followed by wiping the entire surface with the suggested solvent. Proper PPE is a must as most of the solvents you will be using are very dangerous without proper protection. This preparation will give the surface a tooth as they call it for the new paint to grab onto. Many times if this step is skipped or done poorly the new paint will not adhere properly which can and does end up causing the paint to fail in various ways. Again proper repair the first time will prevent you from having to repeat the repair a second time down the road.
The final suggestion I have for you is to use YouTube for the methods that I have briefly described here, as there are many good people out there that go through the process of fiberglass repair and painting procedures. A word of caution not everyone on YouTube gives good and proper advice so doing your homework will be a tremendous help.
Good luck and have fun with your repair, as a good repair is always more satisfying than a bad one.
