I have a bare big-block that's been sitting for 20+ years (and wasn't pristine to begin with). There is surface rust especially on the bores, which are currently standard size. Normally I bring a block and new pistons to the machine shop and have them hot-tank, bore/hone to fit.

But in this case I'm not sure what oversize pistons to order (i.e. how deep are the pits) and if I guess too small, then I'm stuck with them.
Would the machinist be able to rough-bore until it cleans up, then let me know if .020, .030 etc. would be the proper size to buy?
thanks.