Personally I like to use a damp sponge at times.
I find it shocks the iron prior to tinning the tip and I think I get better results.
Metal hardness would be stainless brass and then copper.
If there is a concern whether the irons tip will wear then copper would be a good choice.
Stainless is more abrasive.
Stainless does not let melted solder adhere to it so it does not get loaded with solder.
Brass is more expensive and harder to find here.
I have purchased stainless and copper in separate packaging only.
I prefer using copper pads as long as the iron does not cool down sitting in it.
If it does, the iron will then stick to the copper where there is contact.
Having both stainless an copper lets you decide which to use at a given time.
I use a plastic squeeze bottle with a syringe needle where I need to flood an area.
In the image, you will see a plastic insert, its purpose is to prevent fluid leaking out when the bottle is tipped over.
I like using the brush that comes with the bottle as it can apply flux accurately on the work.
A brush is also better when working on vertical components.
Edit
Don't let the owners bottle know you stole it

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