www.ultimatehandyman.co.uk

Hi,

For general wall plastering jobs in the house I use a rubber mixing bowl.

After several uses, and thorough cleaning of the bowl, it loses its flexibility and round shape.

It looks like it was pushed on the sides and I end up buying another one.

Is there a way to prevent it from getting hard and deformed, such as rubbing with oil, etc. ?

Sounds like a plan, BUT some types of rubber do not "get on" with some types of oil. There is also the question of what is your bowl really made of? then you would need to find a lubricant suitable, then the question would be how much is said lube?
Probably easier and cheaper to buy a new bowl

One of these Click me

Or

One of these Click me

Neither are rubber

I'm talking about a hand held bowl, about 20 cm. across

oh, ok, still almost the same answer though.

Sounds like a plan, BUT some types of rubber do not "get on" with some types of oil.
There is also the question of what is your bowl really made of? (You may think its rubber, but it might be a type of flexible plastic)
Also you would need to find a suitable lubricant. (One that will not harm your bowl)
You can either do trial and error (wasting money on lubrication that does not achieve what you want) Or contact the manufacturer of your bowl.
If there is such a lubricant you have to ask how much is said lube? Will it cost more than a new bowl?
Probably easier, less hassle and cheaper to buy a new bowl

Sorry it is not the definitive answer you were looking for.

Put a bin bag in it as a liner

DWD