I started with a scrap piece of cast iron as a test, examined the results and noted time(s). Put in manifolds let it sit for a few hours judging by test sample results. Washed with baking soda water. The acid was only put into the deepest areas of the manifold where a grinder could not reach. Be mindful of the time allowed to sit! My thought process was to smooth out the roughness rather than take loads of material.
