I agree with Ian.
My suggestion would be to score the surface with a craft knife and then fill with thin layers taking your time buidling up the surface - I would use Mr Surfacer 500 for this as it will grip well.
When it is a little raised above where it should be let it cure properly overnight then sand down with various grades of sanding stick, getting finer as you go, until you see that it looks right. I would then spray primer over the area to check how it looks and then deal with any imperfections using Mr Dissolved Putty and further sanding with fine sanding sticks only. You might have to do the primer/dissolved putty/sanding several times.
The reason for using the Mr Dissolved putty is because it is very self levelling and it gets right into every nook and cranny. There is a lot of shrinkage with this product which is actually helpful but can mean using more than one layer in places.
Once it is fully cured you can re-scribe as needed.
Incidentally - I have tried many different scribers and we all need a needle ended one in some circumstances but for scribing panel lines the very best I have found, by a large margin is the
Tamiya type reviewed in the video below by Phil Flory. The video refers to razor saws but one of the sets is actually a scriber. Be careful, Tammy have different types of scriber and most people are familiar with their 'pea cutter' one and that is not what I am referring to. These are actually p.e. metal for which you use a craft knife handle. Take a look at the review.
