I put some white lithium grease on the threads of the plug. be absolutely sure the grease is wiped away from the breech face and the plug faces. any trace grease or oil will prevent the brown from working with the steel.
A high finish on the barrel will make the brown solution to 'bead up', causing a spotty, uneven streaky, finish. Fix: use no higher than 320 paper, and reposition the paper to expose fresh grit, especially on your final passes. Some builders go right from draw filing to browning.
Be sure to degrease the barrel with acetone/alcohol/KleenWoode etc, before you try to brown. Do not touch with your oily little fingers. Cotton gloves, or hang barrel on a wire.
if you look at a brown finish, it's not glossy. It's satin, the acid having bitten the steel surface. Polishing up the barrel just to etch it back is wasted effort.
A damp box helps with the rusting process. A damp corner in your basement will work well.