I use an old gold enamel paint for my cove line - its not humbrol but another well known make that I can't remember right now. It works well and lasts without any problem.
Humbrol has always seemed to be good quality paint although I've never used it outside. I would expect the gloss to be OK but perhaps not the other finishes.
It's available in three small sizes approx 10cc, 50 cc and 120 cc. Excellent if you only want a small quantity but otherwise an expensive way of buying paint. Also in a vast range of colours.
If you want more than tiny amounts then use a good quality exterior domestic paint. Eg Dulux
Years ago, in the 60's, I repainted a rebuilt racing bike frame with Humbrol enamel paints. I used the bike in all weathers and the finish remained good.
I used it to paint in the "e" on our topsides, and it is fine after several years. Its not as cheap as it used to be, I think the previous time I'd bought any it was less than a shilling a tin!