I am building a kit from T/C and it comes with a walnut stock so would Aqua fortis work on the walnut stock to bring the grain? I know that it works great on curly maple and hope it will do the same on walnut.

I don't know but it would be worth a shot. You can always test it on the butt end of the stock under the butt plate.

It should work I would check on a scrap piece and get a book or do some reading up on it please let us know what you find out.