Weatherproof a galvanised steel garage

I have a galvanised steel garage which is mostly used for storage. Attempt was made to use it or table tennis but the table buckled over a few seasons as dirt, dust and mositure get in this garage.
I would like to weather proof this garage before I buy another table tennis table.
I have seen lots of photos of garages converted for other uses but none looked like it started off as a galvanised steel garage.
Has anyone done this before? If so what is involved?
After weather proofing, I would continue to use for storage and table tennis ie no fancy plans for it as I already have a carport and this garage is set back from the house ie not practical for other use
 
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