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Prepping Rusty Areas Underhood Before Paint
I posted a month ago about what paint to use under hood.  As I got into cleaning my project, there is more surface rust than I had hoped.
I was just going to shoot it with Rust-o-leam rattle cans.  Then I become pretty convinced on the Eastwood 2k underhood black system.

I am well aware of POR 15.  I coated almost my entire interior with POR after I welded in new floor pans. 
I am not excited about seeing brush strokes left behind by POR15 when I spray over it with the finish coats.
Do really have to get ALL the surface rust off before I paint?  (whine,whine) It's a pain in the Butt underhood.

Any other thoughts?  Rust removal jellys?  I've never used them.

Anybody approach an underhood cleanup like I'm facing and figure out something that made it easier.

I'm not looking for perfect show quality, but I do want it to look nice and last.

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Rolloc pads and either an electric or air-powered drill work really well on removing surface rust.

Let me check your shorts!

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Get as much of the rust off that you can, and make sure the surface is as clean as possible before applying paint. You might consider a rust stabilizer to get into areas you can't reach mechanically. You can never get good results over poor preparation.

Get some POR-15 Metal Ready, or any of the other rust removers such as Ospho, Evapo-rust etc. They'll take care of most -surface- rust in no time.

As Midlife mentioned the Rolloc pads work well
As an inexpensive de-oxidiser you could use ordinary vinegar about 50/50 with water to prep the bare metal, spray it on and leave it on for 10 minutes, heavier areas I'll use a scotch brite and work it in, rinse with water, dry thoroughly then epoxy primer
Trick is to get some epoxy (not etch) primer onto it ASAP once prepped
Go ahead and do the following:

1) Remove the brake booster and clean it up.  
2) Remove the wire Harness for underhood
3) remove your fenders

Looks like you have some AWESOME metal to work with.  Use the metal pep mentioned above and spray a nice satin black coating on there.  You will be so happy!!

I would go down the route of sandblasting with that much rust.
If you don't want to go down that route you can use por15 using a spray gun. If you check the por15 website it states the same as long as you dilute with thinners by no more than 20%. My engine was sprayed using por15 engine enamel diluted by 10% so i know its possible

1971 Grandé
I do feel your pain... I'm in middle of a big rust fight for a few months now. I did this on my 73 and and I'll be soon once more enjoying this job on my 71. Smile
If of any help, here's my advise, based on own sweat and tears.

First, you should stop working for your eyes and should focus on a location at a time. I mean by that that it feels good to have the entire compartment sanded, but its of no help. Pick a location epron, cowl vent etc and stick to it till it shines. Light rust generates loads of dust, the very same dust will bloat your sanding paper and you end up spreading a powder of rust instead of the paint/rust bellow. Plus when you work on big areas, the "nobody_will_notice_if_I_skip_this" demon, will hit you! Smile
By the look of the rust, I'd first buy a rotating brush, not the type with long wires, one with twisted wires. They look mighty and one may think its gonna rip off your entire metal. In practice they don't but do 10 times more efficient brushing that the regular long wire ones. Work from lower body to top. (gloves, mask and googles are must must, sound protection is no luxury for this kind of brush).
Once you have the metal free, there will be places where the brush can't go. Use then hand brush and sanding paper.
Rince with clear water. dry it or let it dry. if the surface isn't metal everywhere, fix the bits where there is deposits, often around bolts kept in place.
Using thinner or aceton. remove any trace of grease.
Then use something like evaporust, or the better one from eastwood. These not avail here in europe, so I use Rustico, which is a similar product and gives excellent results on this kind of rust.
These work best if they stay wet. Because you can't do that, prep a bucket with 3 liters of water with 50% or more of that stuff. Using a sponge or rag saturated of liquid, spread the product on the metal, you will see the deep rust turning black and the light one dissolve. Let it wet as long a you can. rince with clear water. For very thick rust, use the product pure with a paint brush. Depending on result, reapply or let completely dry. That stuff goes down to metal pores. When its dry, it leaves an ugly yellowish surface but that ugly layer is also excellent oxidation protection and offer excellent adhesion to primers. Its a ton better than rust transformers as rust is really gone, and you paint on a solid surface, not a transformed layer of rust with rust underneath.
Pick another location and repeat till compartment is done.
Then I'd spray first a thin layer rich in zinc primer or epoxy primer. Then your paint will look good and stay good for a long time.

Its a very dirty work for sure and you will need to sacrifice at least a weekend, but if you do not remove all that rust, no matter the paint quality, it will be a camouflage and you will have to redo this sooner than you think.

+1 on removing the fenders, as there is often rust on top of eprons and likely the need of a small welding session. and the other side is likely corroded too.
also I'd change the brakes lines, they are corroded and its way more easy to do it when you have the space.

73 modified Grandé 351C. Almost done. 
71 429CJ. In progress
I like to use phosphoric acid. Apply it, let it sit 30 minutes, wash it off. Repeat until all rust is gone. It will leave a white residue behind, that you will need to wash and scrub off of the metal before painting. It will not remove paint. It will not work on greasy metal. Wipe down surfaces with paint prep before priming and painting. It is the main ingredient in most rust removers and can be bought cheaply at Lowe's/Home Depot in the paint department.

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"I love my Hookers!" and "Get some Strange" probably have a different connotation to non automotive enthusiasts!
+1 on jeff's post.
I did not insist on the degreasing part while its very important before you apply these chemicals.
Jeff is right: It will not work or a just a little on greasy metal.

73 modified Grandé 351C. Almost done. 
71 429CJ. In progress
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