Much much much patience and hard hard work.
It is totally soul destroying.
that said!
the very best tool for you is a mbx wheel not cheap but good.
or wurth do a wheel that fis a grinder which is far courser than a "wire brush" and retails for about 20 quid.
Beware though press too hard on soft metal and it will burn through.
You dont necessarily need to take everything off just create a good key for what you are putting over it, You must however make sure you thoroughly degrease prior to recoating as no paint ever likes grease and of course depending on the look you are going for replace the seam sealer then topcoat.

If you do it properly you could end up with something like this.









see the rest

https://passionford.com/forum/restorations-rebuilds-and-projects/247462-series-one-rebuild.html

We used a far more durable coating under the arches as series 1 turbo doesn't have archliners. for that we used the 3m spray sealer which is around £10 a bag.
For the underneath we used the upol stuff (gravi tex i think) as we wanted a less stippled finish. you apply it with your bog standard shutz gun, that is asssuming you have access to air facilities.
Believe me if you dont have air tools its alot harder...