Hi Doug; Welcome to permies! As far as your fire box, if your are building a clay /perlite one, it is recommended to locate & purchase ,one or two #50 sacks of fire clay to go with your perlite. Better yet would be to locate fire bricks for your core, insulated and non insulated. Then use your fire clay / perlite mix for your riser. As far as your question about cleaning your clay. No need to make it like pottery, small rocks will not hurt anything. If you are building a cob mass (bench) , then the last inch or two layer of cob on top
should be cleaned well, to make a nice smooth finish. If you are, planning to encase your mass, with brick or stone . Then your cob can be mixed however it comes out no need to be fussy. As far as water glass I would paint it on after your core / riser is formed and been fired. Hopefully you have a copy of the builders guide to smooth your
project. Good luck keep us posted on your build.