[Arran]

 I think you could probably go back and fill the grain again, seal it, and then just coat over it, that's the beauty of using lacquer. I've heard nothing good about crystalac grain filler, Behlens and Constantine's seem to have a much better reputation. I've been experimenting with an Elmers wood filler product, it's water based and can be thinned and cleaned up fairly easily, but on an already sealed cabinet. These cabinets were finished in a gloss lacquer, which was then rubbed out for a burnished look, rather like a piano or a formal dining room table, which was glossier then semi gloss but less glossy then plastic or glass. Semi gloss sort of looks all right but satin looks completely wrong, but many still insist on using it.
 With few exceptions oil stain belongs in the garbage can, it was never used on radio cabinets, now if you want to use aniline dye or fabric dye to darken the bland junk wood I think that's a good plan since it cuts down of how much shading lacquer you need to use. I would be very reluctant to use an automotive clear coat on a radio cabinet, not only is it not designed for wood a lot of that is polyurethane which is murder to get off should something happen to it, or if you made a mistake. Given the track record of clear coat on many cars it's not unusual to see it discolour and peel off, and that's on top of what is supposed to be a non porous surface, wood expands and contracts much more.
Regards
Arran