Ivon

Brick acid will do, the stronger the better. Your better off getting it from a builders supplies then say B&Q as it has a higher conc. To do a port hole etc you should clean off what crud you can then dip it for a couple of hours max then inspect it and give it another clean, it may need another soak depending on how bad it was to start with. Don't leave it in too long or you will start to pit the surface and lose details. The acid does weaken over time so keep adding fresh acid. I have a spare dust bin with lines attached to the bin that I can tie items to and drop them in, makes retrieving a piece of cake.

Just watch your skin and eyes, you may not notice burning strait away.