Yes you can use a gel stain over your existing stain, even if it has been finished. The new coat of stain will add more color to your door. Make sure this is what you want. Undoing this stain application is a real pain.

You have a few gel stain manufactures to choose from. My favorite is Old Masters. They sell a neutral gel stain that has very little color, just the amber color. This is great for “cutting” the gel stains color for a less intense look. Plus Old Masters gel stain is easier to use, in my opinion.

Preparing the door is very easy. All you need is a clean surface to start with. Carefully wipe the door with a damp soft cloth. Don’t push real hard as this could scratch the existing stain and you want it in good shape for a better final look. Once the door is clean and dry you can go ahead and apply the stain. Wait at least 24 hours before applying the clear coat, I recommend two coats of clear.

For more information check out Staining a Fiberglass Door