Lars, I would wait longer after spraying the first coat of varnish, typically I wait 2 days each for the first 2 coats. I hope for you it has properly cured. If not, your varnish will remain soft for a long time, but eventually through the years, it will cure. No need to spray 2 coats of shellac before the varnish, one light coat is enough to block the oils in rosewoods and such, and it needs to be scuffed before the first coat of varnish. I don't know what you call white spirit, but I would use either Epifanes reducer, or naphtha or *real* turpentine as a solvent.
The specks are dust that get trapped on the surface of the finish, the light sanding gets rid of them. It is unavoidable with oil varnish, given the long cure time. To avoid those as much as you can make sure you strain your varnish before pouring it in the gun, and make sure you spray in a well ventilated area free of dust (a conundrum if there is one). Another pitfall is to spray too close to another surface (such as a wall, cabinet, door and so on) and have the air and varnish particles bounce back to your guitar in the form of dust specks. Obviously you need to leave the guitar cure in a dust-free area.
Through the years I have develloped an elaborate, yet simple schedule for oil varnish, what you want is as flat a surface as you can manage to have without sanding through the layers. The Epifanes is not too amber, but if you sand through the wood it will show as a lighter area. For the first 4 coats I just scuff lightly, with 220 with the Epifanes or other softer varnishes. When I feel I have a build, I dry sand (220 if you have a light touch, 320 or 400 otherwise), trying to achieve the flattest surface without going through the varnish. After that I spray one more coat, flat sand to 320, one more coat, flat sand to 400, one more coat, wet sand to P600 and so on. I stop wet sanding at P800 or P1000 before the last coat, which should look "perfect". You may have to go back through a grit or two a couple of times if you get runs. Count on spraying at least 8 coats, probably more, the Epifanes builds fast and is high in solids. I mix it 1:1 with their accelerator, which is nothing less than added resins, and mix that 1:1 with their reducer. I add naphtha or turps as the varnish inevitably thickens in the can. A can of Bloxygen is a must.
It has done most of its shrinking and curing after about 3 days and can then be cut back and buffed, don't go through the last coat to avoid witness lines. That can be minimized by adding a bit of acetone to the mix. It will remain a bit on the soft side for a short while.
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