I used to use Humbrol Gloss clear and Matt clear varnishes, and sometimes the equivalent in acrylics. Nowadays i tend to use Johnson's 'Klear' (Future in the U.S.) for the clear gloss coat, brushed on. Quite often one coat will do the job, but sometimes a second may be required. Just watch for runs/drips. For a matt, semi-matt, semi-gloss finish, I mix Tamiya Flat Base with the Future, to obtain the desired clear finish, sprayed on. Depending how much Flat is added, a dead matt to a semi-gloss clear coat can be made. Takes a bit of experimenting to get what you want, but convenient, and a darn site cheaper than commercial clear coats! Over a metallic finish, it's a good idea to give a clear coat , just over the decals, to seal them. I do it with a brush.
Hopes this helps,
T.