| While I usually do the original dark gray primer color on the underbelly, I still like to get 'em ding, scrape & rust pit free. Here's a couple tips that might help you guys ('n gals )...
1. If you plan to sandblast the pans, degrease them first. Scrape any thick accumulations of oil or grease and in particular clean out the torsion bar sockets. Remove the old undercoating by heating it with a propane torch and scraping. Its a smoky and messy job...be sure to wear gloves and a cartridge respirator. Another method is using a pneumatic scraper and an electric heat gun; although a little slower, it doesn’t produce so many toxic fumes!
2. If you're "going for it", bump out any dings, etc. Fill any serious rust pitting and scrapes with poly putty; it's better and faster than laying on multiple coats of primer-surfacer.
More serious frame rail dents can be pulled by temporarily welding on a flat washer and using a slide hammer. Various pry bars can be useful for working out dents as well.
3. Apply epoxy primer followed by a 2K primer-surfacer. A light coat of a contrasting color can be used as a guide coat and will help greatly with seeing what you're sanding out. Pre-mixed guide coat is available or a basecoat color can be used. I just tint the primer-surfacer darker for the guide coat.
4. A 3" DA sander works wonders! The small pad size gives access to much more area and saves tons of hand sanding time. Some 6" DA's can be fitted with the 3" pad; we have a Dynabrade sander that has a nice small body.
5. Sand starting with 240# (180# may be used if needed), follow with 320#. (Finish with 500# if a metallic bc/cc is planned or if you love sanding!)
6. Blow & vacuum dust thoroughly.
7. I prefer to seal first with epoxy primer before painting. Start by blowing paint into all the frame openings from multiple angles. This will help lock down any dust/crud from flying out of the rails when painting the visible areas.
8. Re-tack wipe and clean. Now continue with the visible areas. I start by spraying the hard-to-reach areas first, such as behind the rear shock crossmember, up under braces, etc. After this you can concentrate on getting a nice finish on the overall bottomside.
9. If you can get your hands on a 2 quart pressure pot setup, it makes the job easier to spray and do a better job. Otherwise, with a gravity feed gun, cup liners can be used that will allow more gun angles without starving for paint.
10. For best long term corrosion protection I epoxy prime first, follow with a single stage urethane mixture of black & white to seal it up. Then for the final correct appearance, use PPG epoxy primer in a mixture of 2parts DPLF40 to 1 part DPLF90. I catalyze this with DPLF402. PLEASE NOTE: cars built at the LA assembly plant were dip painted in a black colored primer. For an alternate method, see the article on using dip gray Zero-Rust as an underbody color.
11. For the red oxide primer overspray (first topside primer color originally sprayed), Zero-Rust aerosol red oxide color is a convenient way to apply it. The factory gray-green sealer color is closely matched with PPG’s DPLF-40. (Last primer color applied before topcoat color.)
Finally, allow the topside color coats to overspray over these. For basecoat/clearcoat systems, using a mixture of basecoat & clearcoat will give a cleaner, less white “fuzzy” edge than the conventional separate base & clearcoat application for the overspray areas. Applying the clearcoat separately also can result in shiny cleared areas over the gray that would look incorrect. Check with your particular manufacturer for compatibility with this technique! PPG reps I’ve spoke with advised a mixture of DBC with DCU clears of 50/50; this mix is then catalyzed & reduced to spray 4:1:1.
I have also done this successfully with Glasurit 55 line & 109 or 155 clearcoat.
I prefer to do the overspray areas first, allow this to dry, then mask off the bottomside edges and finish painting the topside of the car.