TL;DR
Wipe or scrub a cast iron grill press while it is still warm, dry it on the stove over low heat rather than in a rack, and wipe on a very thin coat of neutral oil while it is warm. Cast iron is never soaked and never left to air dry -- both cause rust, and rust is the only real way to ruin a press.
What you are protecting
A cast iron grill press is a heavy slab of bare iron with a handle. There is no coating, no plating, and no stainless layer.
What stands between the iron and rust is seasoning: a thin layer of oil that has been heated until it polymerized into a hard, slick film bonded to the metal.
Seasoning is durable, but it is not permanent. Long soaking, harsh detergent, and prolonged wet contact all wear it back, and once bare iron is exposed to water and air, rust starts within hours.
That is why every rule below is about time and moisture rather than about how hard you scrub. Material dictates method here as it does everywhere in the kitchen, a theme covered in the guide to cleaning kitchen tools without damaging them.
How do you clean a cast iron grill press?
Clean it while it is still warm -- wipe with a paper towel, then scrub with hot water and a stiff brush or a chainmail scrubber, using a small amount of dish soap if it is greasy. Dry it immediately on the stove over low heat, then wipe on a very thin coat of neutral oil while it is warm.
The routine
1. Clean it warm. Fat that is still liquid wipes away. Fat that has cooled and set has to be scrubbed off. Let the press cool enough to handle safely and no further.
2. Wipe out the loose fat first. A paper towel or a cloth, held with tongs if it is still hot.
3. Scrub with hot water. A stiff nylon brush, a chainmail scrubber, or a pan scraper. Coarse salt used as an abrasive with a paper towel is the traditional dry alternative and works well on ridged surfaces.
4. Use a little dish soap if needed. A small amount of dish soap in a quick wash will not strip a well established seasoning. What harms seasoning is soaking, not a brief wash. Never leave a press sitting in soapy water.
5. Do the ridges. A ridged press traps fat in the grooves. Run a brush along the length of each groove rather than across.
6. Dry it on the heat. This is the step that matters most. Set the press on a burner over low heat for a few minutes until every trace of moisture has evaporated. A towel alone leaves water in the pores of the casting.
7. Oil it warm. A few drops of a neutral cooking oil on a paper towel, wiped over every surface including the sides and the underside of the handle mount, then buffed until it looks dry rather than shiny.
What never to do
Never soak it. Even thirty minutes in water can start surface rust and strip seasoning.
Never put it in the dishwasher. Prolonged wet contact plus aggressive detergent removes seasoning entirely and leaves the press rusting by the end of the cycle.
Never air dry it. A press left in a drying rack rusts overnight. Heat drying is not optional.
Never store it damp or in a sealed bag. Trapped moisture against iron is exactly the wrong condition.
Never use bleach on it. It attacks seasoning and does nothing useful, and bleach must never be combined with ammonia, with vinegar or any other acid, or with rubbing alcohol, since each combination releases a toxic gas.
Do not leave acidic food sitting on it. Tomato and citrus break down seasoning on prolonged contact.
Removing rust and reseasoning
Rust is common and almost always fixable.
Scrub the rust off with a stiff brush, steel wool, or a chainmail scrubber. Cast iron is one of the few kitchen surfaces where aggressive abrasion is acceptable, because there is no coating to protect.
Wash with hot water and a little dish soap to remove the loosened rust particles, then dry immediately on the stove over low heat until completely dry.
Then reseason. Wipe on a very thin coat of a neutral high smoke point oil, buff it until the surface looks matte rather than wet, and heat the press in an oven at around 450 to 500 degrees F for an hour, upside down over a lined sheet pan. Let it cool in the oven. Ventilate the kitchen, because this produces some smoke.
Two or three thin coats build a better surface than one thick one. A thick coat goes sticky and tacky instead of hard, which is the most common reseasoning mistake.
Enameled cast iron presses are different and should not be seasoned. Wash them like regular cookware, avoid abrasives on the enamel, and dry them promptly.
Wooden and plastic handles
Many grill presses have a wooden handle, and wood is the one part that must stay out of water.
Wash the iron plate with the handle held clear of the sink, wipe the handle with a damp cloth, and dry it immediately. Never soak the assembly and never put it in the dishwasher.
Wipe the wooden handle with a little food grade mineral oil every few months to stop it drying and cracking, the same routine used on other wooden kitchen tools.
Do not put a press with a wooden or plastic handle in a hot oven for reseasoning. Remove the handle if it unbolts, or reseason on the stovetop instead by heating the plate, applying a thin oil coat, and heating again until it stops smoking.
Storage
Store the press somewhere dry and airy, not in a sealed container.
A paper towel laid between the press and anything stacked on it absorbs incidental moisture and prevents scratching.
Avoid storing cast iron in a garage, a shed, or an unheated space, where seasonal humidity swings put condensation on cold metal. Through a wet Oregon winter, that alone will rust a press that is otherwise perfectly maintained.
Other iron and steel kitchen items follow the same drying discipline -- see how to clean metal skewers and how to clean cookie cutters. Porous stone bakeware has its own separate rules, in how to clean a pizza stone.
Cleaning cookware reduces residue and rancid flavor transfer. It is not a food safety program on its own.
Source: USDA Food Safety and Inspection Service guidance on cleaning food contact surfaces
FAQ
Can you use soap on a cast iron grill press?
A small amount of dish soap in a quick wash is fine and will not strip an established seasoning. What damages cast iron is soaking, a dishwasher cycle, and being left wet. Never leave a press sitting in soapy water, and always dry it on the heat afterward.
How do you dry a cast iron grill press?
On the stove, over low heat, for a few minutes until no trace of moisture remains. A towel alone leaves water in the pores of the casting, and air drying in a rack will produce rust overnight. Then wipe on a thin coat of oil while it is still warm.
How do you get rust off a grill press?
Scrub it off with steel wool, a stiff brush, or a chainmail scrubber -- cast iron has no coating to protect, so abrasion is acceptable. Wash with hot water and a little soap, dry immediately on the heat, then reseason with thin coats of neutral oil.
How do you season a cast iron grill press?
Wipe on a very thin coat of neutral high smoke point oil and buff until the surface looks matte rather than wet. Heat it at around 450 to 500 degrees F for an hour, upside down over a lined pan, and let it cool in the oven. Two or three thin coats beat one thick one.
Can a cast iron grill press go in the dishwasher?
No. The combination of a long wet cycle and harsh detergent strips seasoning completely and leaves bare iron sitting in water, which rusts before the cycle finishes. Wash it by hand in hot water and dry it on the stove.
How do you clean the ridges on a grooved press?
Run a stiff nylon brush or a chainmail scrubber along the length of each groove rather than across them, while the press is still warm and the fat is still soft. Coarse salt on a folded paper towel is an effective dry alternative for grooved surfaces.
Cast iron is simple once the drying habit is set. The stove and kitchen around it take longer. If you would rather hand that off, get a quote or read more about house cleaning services.