IB 911 underbody cleaning and sealing

[Rav]

Hi All,

Im currently having the bodywork on my 911 sorted for rust and rot in the usual places. The car is a 1989 G50 Carerra 3.2.

I was wondering what the best way of cleaning the underbody of this car is and what it should be sealed with after cleaning?

I have heard stories of people pressure washing the underside and rust appearing within weeks due to the removal of sealant?

The car will be a occasional driver, mostly in the summer months.

Thanks in advance.
Rav