Ok heres where my very bad obsession for car correction shines through..
Yest its a MTB, but with a little time and hard work you can definately keep it near showroom fresh.
How deep are the scratches? Can you catch your nail in it? If not they will come out with ease.
Most scratches can be removed if there not down to the primer / metal.
As a first step i'd try something like Meguiars Scratch X, sold in most good car accessory shops such as
Halfords, retails at app £7.99.
Or have a look for some Poorboys SSR (you can buy this on
ebay, and online stores)
This should take the worse of it out, dont be afraid to really work it in, its not a vey harsh abraisive, so you wont burn your paint or anything like that.
A good polish can mask some of the deeper scratches, AutoGlym SRP does a good job.
If it is very deep scratches, how about a paint touch up pen? Take your bike along, and they should be able to mix you a near perfect match.
I personally wouldnt use Nail Varnish.
And i would reccomend waxing it with a good car wax say every 4 weeks.
(i did mention i was a little obsessed

)
This is the sort of results i usually get from the Poorboys SSR, allthough i dont have any pics of anything really deep atm
Before & After