I've used 3M's Professional Grade Rubberized Undercoating on several vehicles now & I would have to say it has held up very well considering the cost (~$10-$12/can at Napa Auto Parts) and it covers well (just don't apply it too heavy between coats or it will run/drip). It certainly won't be as hard to the touch if you were to use a designated bedliner coating, but it will replicate the factory undercoating quite well in that it will stay flexible, absorb some of the road noise from the floor pans, and it won't flake/chip off easily. I would probably every now & then keep an eye on the areas that receive the most contact with road debris on a daily basis (mainly the wheel well openings), but other than that the stuff will last & should protect the underbody from any rust.
I know you're not going to like this part, but realistically the only way to get the existing undercoating off prior to blasting the rusted area(s) is by using a combination of chemical or mechanical work to remove the majority of it. Sandblasting, while very useful for cleaning rusted metal, does not work well when attempting to remove any tar or gooey material (it actually just makes it worse if you ask me 'cause it makes it even more gooey & consequently harder to remove). What you'll end up having to do is either remove it chemically with something that can eat away at the undercoating's bond to the metal
OR you'll have to do it mechanically.
I haven't really used the chemical approach to it before, but I can say that the mechanical way is very effective. Unfortunately, it really is time-consuming & somewhat tedious in hard-to-reach areas that your tool of choice can't effectively hit. When I did my Challenger's underbody in preparation for repairs & paint, I used a combination of a wire wheel cup on a 4 1/2" angle grinder (make sure you have PLENTY of layers on to cover your body & safety goggles & even a face shield are a MUST!!!), otherwise you'll end up with a ton of metal splinters all over your body as they continue to break off the cup at very high speeds.
The other tool was very handy, though I certainly wouldn't have had access to it if I didn't personally know the shop owner ($$$$). It's Snap-On's "Crud Thug", which was a pneumatic tool that has fiberglass(??) flapper discs on the end of a super light, glorified die-grinder looking tool (very little hand fatigue!!). That thing worked amazingly well getting into any of the small crevices I missed from the night before with the wire wheel cup, but just know that it is quite an expensive tool (especially if you're only going to use the thing once or twice).
Actually found it on their website (it's gone up some, lol):
http://buy1.snapon.com/catalog/item.asp ... ir=catalog
:shock:
I wouldn't even attempt to sandblast the underbody until you have the majority of the undercoating removed or else it will just turn into a gooey mess that's even harder to clean off than before (ask me how I know, lol). :twisted:
Since you plan on spot blasting the underbody, I would probably look at getting a quality product down as your base (POR-15 or an Eastwood base coating, such as chassis black, etc.), allow ample time for it to cure properly (whatever the manufacturer's recommendations are depending on which one you use), and then top-coat it with some 3M rubberized undercoating & it should stay clean & rust-free for many years. 8)
Good luck with it!