Sheets of metal have a lot of issues, unless you bring hugely THICK sheets of dense metals. All you do with thinner shielding is turn your alpha/beta risk into

Sheets of metal have a lot of issues, unless you bring hugely THICK sheets of dense metals. All you do with thinner shielding is turn your alpha/beta risk into a re-radiated gamma risk. Plus, dense shielding is heavy... and therefore expensive in terms of its Delta-v.