This is a problem caused by a poor job of sealing the lip of a cast iron sink. Of course cast iron is made with ferrous metals that can rust. If you leave an iron skillet outside in the rain for a week, it will quickly rust. An iron sink, even though porcelain coated, can cause rust stains near the granite cutout, usually because the surface/lip if the sink that contacts the granite is not glazed or coated with porcelain.
A very good plumber, when setting a sink of any kind, should shoot a solid bead around the cutout of Silicone caulk, not latex, and drop the sink into the solid bead of sealant.
Latex caulk is water based, and can cause rusting on the edge of an unglazed cast iron sink. By dropping the lip of the sink into a bead of silicone caulk, you can likely prevent much of the problem from occurring in the first place. Of course this is much harder to accomplish than simply shooting a bead of caulk around the edge of the sink after it is installed.
How to Remove Rust Stains from Granite
Once you have this particular problem, the best solution is to have the sink removed so you can properly treat the granite with a rust remover. I suggest using the HMK R77 Rust Remover, followed by neutralizing the area with HMK R55 Intensive Cleaner. We offer these two products in a convenient kit HERE
Applying the Rust Remover 50/50 diluted with water is suggested, allowing the solution to dwell on the surface for 20 minutes. Wipe up solution and rinse well with water, followed by a Neutralizing Solution of R55 in a 1-part to 6-parts water solution. Rinse well again with clean water, and allow to dry. You should see the rust stains lightening or depending on how deep the rust has penetrated, it may be gone. For deeply penetrating rust stains, you may need to apply R77 full strength.
Always test the acid solution in an area under the lip of the sink. You should mask off the visible area with painters tape or duct tape. NOTE: Some stone that is commercially called a “granite” may not be a True Granite geologically, and acids may etch the polish. Testing is always recommended.
To further prevent rust stains, and stains of all kinds, it is essential that you seal your granite, marble and all natural stone with a quality Impregnating Sealer. Please see the following link for details HERE
If you are uncertain of the nature of your stone, please contact us for assistance. Send pictures by email HERE or contact us by phone at 800-380-6881