Aluminium was chosen for:

• Weight.
• Ease of construction.
• Low maintenance.

Why paint a surface unless to protect that surface from the elements?

Aluminium needs no such protection and Horst decided STAR TREK
would remain in it's natural state.

Star Trek by Commercial Catamarans.

Those who have sandblasted and painted a steel boat (and have continued to do so in bits and pieces over a number of years), will appreciate the complete lack of attention and $$$ needed to keep an aluminium boat at sea and looking good.

The only maintenance required, apart from the normal maintenance on machinery and fishing gear, is slipping and anti fouling the bottom. Forget about the topsides.

Don't spend time, profit and hard work maintaining the hull.
The saving in time and money on a boat this size are huge.
Obviously it is possible to paint the vessel if required,