A lot is dependent on how bad it is and how much preparation work is required
Once it's in the seat pans It's almost impossible to fetch it down to bare metal without dropping the axle, and media blasting, painting over is a bad idea
The normal solution Is
In summer get a underbody wash done
Then wire brush the loose rust and paint off
And slap some axle grease all over
If it's not too bad it will probably out last the rest of the car