Can I use a Potentiometer to reduce 18vdc to 14.5vdc for a 36W filament?
Don't use a voltage divider or any other linear method of reducing the voltage. Use a buck regulator. Good ones are usually about 95% efficient at converting from a higher voltage to a lower voltage so with a load of 36 watts, the power dissipation might be ~2 watts.

Here's what I mean (and note that it can be easily made to run from 18V in to produce 12V (or 13 or 14 or 11 or 10 volts out): -