Need help with finishing an oil painted guitar?

Hi all,
I am in desperate need of some expertise and hoping someone has tackled this problem before, hopefully with some level of success. I have an electric guitar which my artist cousin has painted with oil paints. Now I am trying to figure out the best way to give this guitar a nice clear gloss finish that is going to be relatively durable. I initially thought poly over the top but wasn't sure of compatibility and went to an artists supplies shop and they recommended a spray picture varnish, which looks OK, but I am concerned about its durability. I have searched and asked around and found a bit of conflicting information :? and am at a loss as to what to try, as I dont want to ruin the artwork underneath.
Any ideas on how to get this guitar finished? Anyone had luck with poly (brush or spray) over oils?
Much appreciated for any help!

Test the compatibility, get some of the oil paint from cuz do a rendition of Rembrandt and paint it with the clear see what happens.

You need to make sure thr paint has hardened properly, it can take six months or so for this to be acheived with oil paintings, then go to a art supply store they have finishes designed for oil paint. Finishes for oil paintings are not meant to be permanent, the are designed to absorb all the nasty's in the air plus dust and grime instead of the painting, after many years when the painting is looking dull and dirty it can be restored by removing only the finish leaving the painting unscathed

mmmm, 6 months hey, don't think its been that long yet, maybe 3, but it 'feels' relatively hard. It was pretty thin coats on the guitar so maybe it could be ok? I think i will take your advice harry and do a small test piece. There are some areas painted on the guitar that will be hidden so will try it out on that first.
Will keep you all updated with progress!

Okay, so on the weekend I was able to do a test piece on a concealed area of the guitar and it turned out fine. So I have since sprayed 9 light coats of Polyurethane on the guitar, waiting about 12 hours between each coat and its coming along well. Sooo far. :)
I am going to leave it now, maybe 10 days or so to really cure and harden then hit it lightly with the random orbital with maybe 400 grit. Try to start smoothing it out a bit.
Hopefully i have enough clear coat on the body now that I can cut down a little and not hit paint, fingers crossed.
After that, more coats and sand again. Ive read some great things about EEE-Ultra Shine too and am keen to finish the guitar off with that.
Now, I try and wait patiently. :)

Don't think i would use the sander. I would try 600 wet/dry sandpaper by hand, you'll have better control using it wet.

Bit of an update here!

So, after letting the poly dry for a week, I sanded down with Wet/Dry 600 grit (thanks Mobil Man) and it worked really well, I did hit through to some of the artwork in some high spots but it was mainly just where block colour was, so no big deal.
Following that, I repeated the process of laying on another 9 coats and letting it cure for a week and I then sanded it back with Wet/Dry 800 grit, and it is really smooth now, BUT you can see in the attached photos that it is really visible where I have cut through some layers of Poly at varying heights (because of the varying height of the artwork beneath it) and its really visible in the light.
How do I go about getting rid of this? I figure going smoother will do little to fix the problem because its still going to have the same visual effect. And I know that if i put more layers of Poly on and then sand back, its still going to be the same?

I would ideally love to have a very smooth glossy finish, but am unsure how to get there from here.
FYI, I re-coated every 12 hours, to ensure that each layer chemically keyed into the layer below as per some advice on another thread.
Any ideas?
Thanks in advance for any help, I've included some other pics of the work in progress too:)

Righto so seems like there may not be much I can do here, but I really don't want to admit defeat on this guitar just yet.
So I was thinking of maybe doing the following to try and prevent the witness lines from appearing when sanding back.
So far my re-coats have been every 12 hours, once the previous coat has dried, but not fully cured. What if i was to re-coat every 2 hours or so? Do you think this could help the layers of Poly blend together so when I do my final sanding it wont be cutting through layers leaving the witness lines?
Again, it would be the Oil based gloss poly in a spray can.
If this doesn't work then I think I could be out of options, besides putting a final layer of poly on to cover the witness lines, but then its likely to not be as smooth as a very fine wet/dry paper finish,
Thanks again all, any tips would be welcomed :)