So we have a lovely white porcelain farm sink in our kitchen. However, the bottom is becoming quite discolored and dingy. I don't want to use anything harsh and chemically on it and I don't want to damage it but I do want to restore its lovely whiteness.
Use a damp (more on the wet side) sponge and sprinkle some baking soda on it. Wipe the sink with this. You can also make a paste of the baking soda with water first and then use a damp sponge and then wipe.
i scrub with baking soda using my dish cloth. then, i lay it flat in the bottom (drain closed), add white vinegar. once it stops fizzing, add peroxide and let it sit for a long time while i'm doing other things. i always come back to a fresh white sink and clean dish cloth!. good luck!
Amy - baking soda is very soft on the MOH's scale (the hardness of minerals - for example, diamonds are very high on the scale and can scratch just about everything). It shouldn't scratch your porcelain at all, since baking soda dissolves a little bit with a little water and dissolves completely with a lot of water.
If you still think it may be the baking soda, check to see if you have the kind in the shaker bottle sold in grocery stores meant for stainless products and cleaning, you may want to downgrade to the food version of the baking soda, which has smaller particles. Again, I doubt that your baking soda will scratch anything. It's safe enough for enamel on teeth, which is relatively delicate too.
Make sure your sponge is not abrasive or has any scrubby pads in it.