Can mederma work for chemical burn scars?

Most likely not: Mederma is an over the counter scar therapy agent best used for post surgical scars or minor scars. A chemical burn scar was probably third degree (deep) and will thus need more involved measures such as steroid injection or scar revision.

Not a fan of Mederma: I am not a big advocate of mederma. The active ingredient is allium cepa (onion extract) and there is weak evidence to suggest that this is beneficial but the research behind medierma is some what not convincing. While it will not hurt unless you have a topical allergic reaction, it probably will not help. You are better off to try silastic sheeting or silicone gel + sunblock.

Better to go with: Silicone sheeting and a custom fitted pressure mask. There's lots of research in the burn literature to show this is an effective method for reducing scar thickness and improving the appearance. Mederma - not so much...