Allan's old bottle of Humbrol gloss varnish

[Allan]

well, do you think it's useable?    it's thinnish, but a little yellow and I bought it years ago     worth a try, thinned and airbrushed, do you think?

[NARSES2]

Allan I'm not sure about that yellowish tinge ?

If it were me I'd try it out first by brushing some on a piece of scrap plastic or at least somewhere on the model it's easy to fix.

Just my tuppence worth

Chris

[Captain Canada]

I'd thin it first and then do as Chris suggested. Even if it dries a wee bit yellowish it might be worthwhile for some schemes 

[zenrat]

I'd guess it'd be OK as long as you allow for the yellowing.  I.e don't put ti over white and expect anything other than a cream finish.

[Rick Lowe]

You could use it as a sealer coat under the primer - a friend made the suggestion (though he uses Klear) that this technique provides a uniform surface, especially under metallics, so you don't get a change in sheen/colour between plastic and filler/s.

I've used it myself, and while it seems to work, TBH I'm not sure if it's necessary - especially with the modern fillers.

FWIW (Shrug Emoticon)