Didnt know where to stick this as its not really technical.

Im in the process of rebuilding my S2 ERST (see my rebuild thread) and hopefully getting it back from the welders this week.

I havent got a ramp but have got axel stands and trolley jack so as it has had a patch in inner wing,near rear arches and bits and pieces done on sills i want to completely rust proof it as best i can when it comes back.

What is the best stuff to use and how do i go about applying it?
Please bear in mind im not going for a concours standard car but just a car that wont rust anymore (or at least hold off the rust as long as i can).
I dont want to eat my dinner off the underside or anything but obviously want it to look nice enough so if someone came to view it if buying it they would say it looks clean!!!
I want to rust proof as much as i can (engine bay - though engine still in and dont want to take out just yet),underneath,in arches,under bodykit and bumpers (note these have now been stripped off) and inside (interior out inc. carpets and insualtion except dash board).Think it might have bits of underseal already underneath and places where its welded so can i just go over these?

I was thinking rollering (on the bits i can) smooth white hammerite (and using a brush for the little bits) underneath,inside and inside arches and under bodykit (car is white) or will i need to put something down 1st before going over the top with smooth white hammerite? I think using the smooth white hammerite will give it a factory clean look or am i wrong?

Any advice will be handy including best places to buy and best techniques and prices for anything i need.