Before applying the fuel tank sealer, you will need to plug up external holes, for this there are many ways of doing this with simple household and Garage items.
For the fuel tap, you can use a bolt, putty/play doh, Cork/Silicone or Rubber plug, anything that will extend into the fuel tank, so that the sealer only goes around the bung and not cover it.
Any Breather or outlet port can be sealed in the same way.
If you have any damage or holes in the tank, cover with duct tape to stop the sealer coming out, when the sealer has hardened, you can then repair the outside as the tank sealer will be solid and can be filled over, prepared and then painted.
On Motorcycle and cars that have fuel gauge linkages and electric inside the tank, you will need to remove these before you apply the sealer, remembering to plug any screw fixings holes that are there, the existing screws will be perfect for this.
If your fuel tank has good paintwork and you are worried out spilling any cleaners or sealers on it, wrap the tank in layers of Cling Film before starting, piecing holes in any inlet or outlet holes, this will protect you tank.