way easier than what is stated, sandblast as well as concievably possible. .....
Then prime with a epoxy primer, (even if it means brushing around by hand).
now the fun.. buy a quart of spray on bed liner (yes , the stuff used in pick up beds) This IS catylized, so you only have about an hour at best to apply it.
I have done it with a body schutz gun, and a tip bent so that I can access all points , even upside down. Once done it is a MUST THAT YOU clean all over spray from anything surrounding, cause it will stick FOR GOOD. You will now have sealed up the inside plenum of your cowl panel, no more rust, no more leaks .
Apply liberally.