Will these snips handle stainless steel or just mild steel?

These are rated for sheet metal work — mild steel, aluminum, and thin materials. Stainless is significantly harder; test on scrap first to confirm your material thickness and hardness before committing to a full panel cut.

How do I keep the blades sharp after heavy use?

Hand snips dull with repeated cuts through thicker gauge material. A whetstone or blade sharpening service will restore the edge. If cuts start pulling or tearing instead of shearing cleanly, that's your signal the blades need attention.

Does the thumb lock stay secure during a long cutting job?

The lock holds the blades shut for storage and transport. During active cutting, you'll open and close the snips repeatedly — the lock isn't designed to stay engaged while cutting. It's a safety feature for between-task storage only.