I’m not sure the application, but probably fire could be useful. Just being silly. Ignore me.

Specifically for toilet bowls, if you find that a stain won’t shift even after using a normal toilet bowl cleaner solution and a normal toilet brush, don’t try to brush harder as this will scratch the porcelain surface. Also don’t try something abrasive like steel wool or the green/blue side of a kitchen sponge.

Instead, what you want to do is dissolve the stain. Others have suggested CLR and that might work. But if not, then you can obtain “acidic toilet bowl cleaner”, which contains hydrochloric acid, aka muriatic acid. This will remove most anything normally staining a toilet bowl, but make sure you handle it sturdily and carefully; it can mess you up. Gloves and eye protection are highly recommended, until the bowl is brushed, the stain is gone, and the bowl is flushed. Turn on the bathroom fan or open a window for ventilation during and after cleaning.

In general, for difficult cleaning jobs, don’t try to clean harder, but clean smarter. If you’re putting your whole body weight into a towel or a brush, there’s almost certainly an easier way. Good luck!

I’m really surprised at how many people have their why not to mix cleaning supplies wrong.

Bleach + ammonia can make chloramine gas by the bigger problem would be making NCl3 and blowing your toilet up.

Bleach + acid(like cleaning vinegar) is what makes chlorine gas that will auschwitz your bathroom.

Bleach + rubbing alcohol actually makes chloroform, which is fun.

What kind of surfaces do you have? For example, I’ve found CLR is good for grout and tiles, while a basic bleach-based spray is good for porcelain and fiberglass. You can also mix in an ammonia-based cleaner if you want to recreate the Third Battle of Ypres in your bathroom.

For the bathroom, I swear by my steam cleaner. No chemicals, just hot steam. If that doesn’t get it, vinegar is good escalation. Soak the spots in vinegar and you should be able to wipe it off easy. Maybe screw you shower head off the wall and let it soak over night.

Bleach is awesome! Mix it with some water, use a rag or sponge and scrub. DO NOT mix anything else with the bleach. Bleach is basic, and a lot of other cleaning products are acidic. Mix a strong enough acid with a strong enough base and you get mustard gas. It doesn’t smell too great, on top of other issues. If you want to clean with bleach plus something, be safe and buy premade cleaner with bleach.

EDIT: I should also say you can’t use bleach on everything, it can damage some things. Always test a small, hard to see area before you clean a surface with bleach. For the stuff that you can’t get bleach on, you can use white vinegar or all-purpose cleaner.

I feel like it’s worth mentioning here:

DO NOT MIX BLEACH AND AMMONIA. You can die.

At the very least, I’d recommend you:

• gloves - because you’ll get really close to that gross shit. You don’t want to touch it.
• a sponge - it doesn’t need to be new; your old kitchen sponge is enough, just don’t use it again in the kitchen. Use the yellow side to spread the cleaning agent, and the green side to remove obnoxious grime stuck to something. (Do it gently, and only with a really old sponge, to avoid scratching the surface.)
• a bucket - mostly to mix some soap and water.
• a dry rag - mostly for finishing/drying. A cringey old shirt that you won’t be using again is usually enough.
• toilet brush - don’t use the sponge to clean inside the toilet bowl; you’ll be spreading the bacteria from your shit and piss to the rest of the restroom.

Everyone has the cleaning agents that they swear upon, so look for something that works for you. For me it’s

• alcohol vinegar - to get rid of that brown crust in the sink (water in my city is hard as a brick) and around the shower drain. I usually apply it, wait a few minutes, then use the sponge to scrub it a bit. Then I remove the vinegar with the rag.
• bleach - exclusively used inside the toilet bowl. I squish some bleach there, then scrub it with the toilet brush, then flush it off, making sure that there’s no bleach behind.
• disinfecting agent - I squish a bit of that inside the toilet bowl and just leave it there. It smells good, and it gets rid of the bacteria.
• an ammonium-based cleaning agent - I squish it on obvious grime on the walls (except the above), then scrub it with the sponge.
• soap and water - to “wash” the walls with the sponge.
• plain water with some disinfecting agent - to rinse it. Then I just remove the excess water with the rag and let the restroom to dry naturally (with closed doors otherwise my cats will step on the bathroom, step outside, and now I got to clean the bathroom again plus the corridor and furniture).

Important detail: do not mix any two of the cleaning agents that I’ve mentioned. Specially not ammonium and bleach.

For reference, the disinfecting agent that I use is called “pinho sol”, but I have no idea if it’s sold outside Brazil. You probably have some similar product wherever you live.

Elbow grease.

Bleach for pretty much every surface compatible with it. CLR for the shower itself (or a bathroom cleaner specifically for scale and soap scum). Simple Green (or whatever the equivalent is where you’re from) for every thing else. Buy a decent non-scratch scourer (or a couple) as well as some magic erasers to help with any scale that’s resistant to removal. Oh and glass cleaner for the mirror and maybe shower. Nitrile gloves never hurt if you’re dealing with heavy duty chemicals either. Keep the bathroom well ventilated and never mix chemicals.