If a crack appears in your marble shower, you will need to repair it immediately to prevent damage to the floor or walls from water that penetrates through the crack. Mold and mildew can develop and the walls and floor can rot leading to expensive repairs.
Step 1 - Prepare the Area
Make sure the marble around the area to be repaired is clean of all soap scum and any cleaning product residue so the epoxy will adhere properly. If you are repairing a shower wall, put a newspaper or a drop cloth on the floor of the shower to catch possible drips.
Step 2 - Apply the Epoxy
If you are using clear or neutral epoxy, it can be applied by manufacturers' instructions directly to the crack and smoothed with your rubber scraper. Be sure the entire crack is filled and as level as possible.
If you are going to tint the epoxy, mix the clear epoxy with the marble dust or tint medium to the desired color. You can use a paper plate and a plastic knife to mix it. Use something you can discard. When you have achieved the color you want, apply it to the crack in the same manner as above.
Step 3 - Finish the Repair
When the epoxy is totally dry and hard, use a very fine-grit sandpaper to sand it even with the rest of the marble. Be careful to not scratch your marble with the sandpaper.