When checking out the new owner on the workings of the E300 last week, I noticed a good deal of surface rust (staining) in there on the floorboard, which developed when I added water to the previous battery 4 +/- years ago.

The rust is over the car's color, Smoke Silver, so it's purely rust colored debris or topical powder only at this stage. But I want to get vigilant and busy on cleaning it thoroughly this time!

My Q is how to do a complete cleaning of the cavity, and perhaps coating it with something to discourage even the surface rust in the future? I cleaned with baking soda and water last time I changed the battery out years ago. I also bought a new rubber/plastic grommet that plugs the hole to drain, since the original was worn out 4 years ago. The replacement is in good condition still.

My 2nd Q is whether one should ever add water to these batteries? I read somewhere that you should not - hence why it over flowed into the cavity - however it did not drain out the overflow clear plastic tubing to the exterior.

Thanks for any expert insight or been there, done that with this model car!