I use nitrate on rubber models and gliders, butyrate on gas models.
You can use butyrate over nitrate for final finish but not nitrate over butyrate. Some of the older gas model instructions I have that are tissue covered suggested two coats of nitrate and one coat of butyrate for fuel proofing. I guess there are a few of us left that still cover with tissue.
If it’s a none gas plane than it’s 5 coats of Nitrate cut 50%. On gas models with 25% or less Nitro it’s 3 coats Nitrate cut 50% than 2-3 coats of Siq Butyrate Lite dope cut 50%. On gas model fuselage’s always coat the front with two part epoxy or something similar cut 50% for HOT fuel proofing of the fus.