Good luck with your weathering project

hopefully it turns out nice.
That said I'm going to go with the general consensus that Eldar wouldnt be caught dead in a heavily damaged and weathered vehicle, They operate shields and holofields as protection from being hit in the first place, in the event that those safeguards fail I would expect the grav tank to be going down as they dont have the look of being able to sustain heavy damage and carry on like an ork vehicle does.
I could only imagine that any damage seen on an Eldar vehicle is going to be really freshly made during the battle being played and that theyd never turn up on something thats nackered already.
If you wanted to fix up your tank and make the lines clean, remove the cracks etc, then I suggest milliput as a filler, its very easy to sand comapred to greenstuff.
Strip the model first down the as much bare plastic as possible and then go about glueing broken pieces together and filling cracks/holes with milliput and then sanding it back smooth to hide the repair work. Adding multiple layers if its still visible after one pass. (the trick is to over fill and then sand it back rather than to try and fill it and shape perfectly in one pass.
Another idea might be to make terrain from it in way of a wrecked grav tank, it could be a good objective marker or have misisons based around it, or simple be cover on the batlefield
Forge world are doing a crashed thunderhawk realm of battle piece, something like that (in a much less grand scale) might work well.