Next, we sanded the entire engine compartment with 320-grit wet/dry paper wrapped around a rubber sanding block. There's no need to sand down to bare metal, but it is important to sand everything that will be painted.
This gives the new paint a textured surface to stick to-note the darker, unsanded areas above the steering column in the accompanying photo.
During this process, some rust bubbles were unearthed in the area under where the battery tray had been. The rust didn't stand a chance against an 80-grit disc followed by several passes with a red Scotch-Brite disc. Then, the edges were feathered by hand with 320-grit paper.
Any areas of bare metal must be treated with an etching primer or a dedicated direct-to-metal primer before proceeding. For this project, we sprayed any bare metal spots with a couple of light coats of SEM's Self Etching Primer.
Next, we masked and covered the car. We also took the time to cover the adjacent cars-you'd be surprised how far overspray can travel. Then, the entire engine compartment was blasted off with compressed air followed by a dousing of Eastwood PRE cleaner and paint-prep solution. Even though this is just an engine compartment, dirt and dust will still look ugly embedded in its new paint.
Proper mixing of all materials is crucial or funny things will happen to your paint job over time. Follow the directions on the can closely, or better yet, follow the procedures on the product's technical data sheet if you have access to it. DuPont's 2840S sealer is a two-part urethane that must be mixed with an activator at a 2:1 ratio. Be sure to stir the mixture thoroughly and strain as you pour it into the paint cup.
Check the air pressure before spraying. Most HVLP (high volume, low pressure) guns are designed to work with about 8-10 psi at the gun cap. Accounting for the pressure drop through the gun, this equals about 26-28 psi at the heel (i.e., inlet) of the gun. We used a "cheater valve" regulator on the air fitting; however, if you plan on doing a lot of spraying, a wall-mounted regulator to connect your air hose into is preferable. They provide more consistent air pressure and, since they often incorporate a filter and water separator, that air is clean and dry.