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Okay guys, after a very interesting time playing with chemicals, sandblasting, and painting, here's what I've learned about removing heavy (or minor) rust from body panels that will be exposed to salt spray.
Caveats: I have not tested this solution yet long-term but according to the discussions I've had with both the primer techs and the people who make the phosphate pre-treatments, this *should* work the best.
1. Equipment needed:
- Sandblaster (the 50 lb. kind from sears works great)
- Air compressor (at _least_ 1 hp, I would say something able to sustain 9-11cfm at 90psi is the minimum for continuous blasting, otherwise its blast... wait... blast... wait)
- Scrub brushes, CLEAN rags, etc...
- Paint gun with 1.4-1.6mm tip. I chose a gravity feed HVLP model from harbor freight for $50 and it works beautifully
- In-line water trap + water filter for your gun
- Air nozzle to blow dust off things
- Depending on the area and your patience, at least a 50lb bag of sand (will maybe do a wheel well) or more. I cycled 3 50lb. bags and ended up after having lost a lot of sand to dust and the environement, with about 1 bag worth to sweep and reload). I used 60-40 silica sand (requires a respirator, full face mask, AND eye goggles -- you can get better protection equipment if you are doing a large area. Don't skimp on this - silica dust is dangerous if inhaled)
2. Chemicals needed:
- Primer: PPG DPxxLF or 97-14xx (xx is for color choice) :: get this at Auto Color or other PPG jobber. It will run about $50-60 a quart, but 1 quart will cover almost the entire underbody and engine bay and wheel wells with 2 coats, trust me).
- Phosphate pre-treatment: Must for Rust (by Krud Kutter, I think) worked the best. Get this at Home Depot or other hardware stores. I tried DuPont 5717S and 5718S but you'll see why I didn't use that shortly. $8. Henceforth called MFR.
The concept is simple: sandblast to white metal (despite what you would think, it is NOT shiny... its a dullish but very clean, soft-looking white-ish surface), then prime immediately after blowing all dust off. Do NOT use a tack rag or otherwise wipe the surface with anything (this according to both DuPont and PPG). However, there are a LOT of areas on this vehicle where you cannot get sand to remove the slight rust film. What to do?
If there is ANY residual rust on the surface (e.g. in the pits of a weld line, on the backside of a bracket that is welded in place, etc...) you MUST neutralize this rust before you prime. Even if you use the best primer (which those are), it will spread, albeit slowly. This is where the tricks come in. If you follow the directions on MFR or the DuPont equivalents (the 57* chemicals) you WILL GET FLASH RUST. This is immediately forming rust on your fresh surface (nooooo!!!)... Bad Thing. Do not let water get anywhere near your surface. Instead, if there is ZERO rust (a completely freshly blasted, usually smooth surface) then prime within an hour. If there is possibly ANY rust left (in tight corners, brackets, welds, etc...) then spray with MFR, wait until the rust has turned black instead of brown, and wipe the MFR off before it dries. If it dries quickly on you, respray with MFR to rewet the area, then wipe that off. Use as little as required to do the job. If any chalky powder forms as a result of too much phosphating still left, you CAN use a tack rag to remove it before priming. Then prime within 20 minutes. Scrubbing the slightly rusted surface with a nylon brush while it is wet can help accelerate this conversion process.
This should yield a solid, clean, long-lasting primer base for you to paint or undercoat.
How it works: The MFR has phosphoric acid in it, which dissolved loose rust, and converts the tightly-clinging rust to a different compound (a more complex iron oxide). This "black rust" is neutralized, will not spread further, and is solid enough to prime (assuming you've already blasted all loose material off or away from it and this is trace rust remaining). However, the phosphate process, if it dries on the surface, leaves a powder which can damage the priming chemistry, and prevent good bonding to the raw metal. If you rinse it off, as the directions say, the pits in the blasted metal will retain the rinse water, despite your best efforts to dry it with heat or air, and will begin to rust almost instantly (because its super clean steel and water -- instant rust). So the light spray, with a good wipe BEFORE it dries, removes as much of the phophatizing as possible. Removal of any residual powder completes the process, and the trace phosphates left should not (according to PPG) cause a problem. I have the entire underbody of my car coated in this fashion, and the primer (so far) appears to be VERY solidly stuck to the surface after 2 days of being on there. I will try to keep this post updated if there is any negative progress, but according to the combination of wisdom from the various technical support people I spoke with, this should be the proper way to prepare a surface.
NOTE: If you are using PPG 97-14xx (I used DPLF), it is designed to be applied directly over tightly-bonded rust. However, according to the manual for this paint, it WILL chalk, lose gloss, and otherwise visually deteriorate over time. It will NOT lose its corrosion protection, however. So if you don't mind this happening (e.g. transmission tunnel or whatnot) this might be the better choice. I found the tranny tunnel to be the easiest place to get 100% of the rust off, however, and the rear underbody of a convertible to be the hardest.
And yes, for those of you so wondering, the 'vert does have a rear strut bar already, in the form of a significant piece of metal framing that runs on the underside of the body panels at about the top of the shock towers. No BS there.
None of this comes with any warranty, its just my experience and the summation of what I received from Tech support. However, I'll be glad to answer any questions if anybody has them, even if this thread gets really old.