john royce wrote:

If I soaked the cast iron one like that I think it would quickly turn to
rust. So would anyone have any experience of easily cleaning a rough
surfaced cast iron grill after use? Thanks.

Cast iron is ideal for cleaning with sodium hydroxide. You need a
shallow tray large enough to take the grill, and a large dose of clue.
If you've not worked with caustic soda or any form of chemical before
this may not be for you.

You add caustic soda to water, never the other way around. It will boil
and splash. Splashes on skin must be washed off immediately in cold
water, some recommend using white vinegar but that's not necessary IMO
if you rinse under free-running water.

Splashes in eyes can result in you losing your sight so you need to wear
goggles (at least) or a full face visor. You will also need chemical
resitant gloves (NOT rubber) and a devent quality waterproof apron, PVC
or polythene.

Any splash in an eye and you need to wash in huge amounts of water and
get to a hospital straight away, but the damage is likely to be
permanent.

You need about 25g of sodium hydroxide per liter of water. This solution
is highly corrosive. I would make up the solution in the tray ensure the
sodium hydroxide has dissolved by stirring gently with a wooden spook or
paddle then lay the grill gently into the solution and leave for at
least four hours, preferably overnight. You can, oif you are careful
scrube the grill very gently with a long handled nylon brush to loosen
the grease.

Remove from the solution, wash well in water and dry immediately, if you
can leave it in an airing cupboard to dru off properly. You'll then need
to "season" the cast iron by wiping it over with edible oil. I'd then
store it in oil-soaked newspaper.

TBH it's a lot of messing about and strictly not necessary.