Painting clear parts

Most gloss paints are difficult to work with and will leave streak marks if applied by brush. I've never been satisfied with the results of the lighter Humbrol gloss colours, especially when applied by brush. I'm not suggesting there's anything wrong with them; just that I find them hard to work with.
For best results, use a light-coloured matt (or "flat") base coat. The coarser pigment in a matt paint will give the gloss coat something to "bite" into. Use white or very pale grey for a base coat to a gloss white top coat. As Jerry suggests, your best results will be from spraying, either by spray can or airbrush. Nevertheless, you may still require a couple of coats for good coverage. It's best to spray light coats to avoid runs & build-up on details, allow each to dry thoroughly and give a light rub with fine wet/dry or a moist Scotchbrite pad before applying the next coat.
Alternatively, follow Jerry's suggestion of using matt paints and simply finishing with a top coat or two of gloss varnish.
Just a small tip: add a very tiny amount of blue to your white paint (a small drop to the Humbrol tinlet should be enough). Not enough to visibly change the colour, but enough to help the coverage. This will also offset any future "yellowing" that white paints are notoriously susceptible to.
Good luck, James.

I mask off the areas to remain clear, spray with gloss topcoat to seal edges of tape, then prime the piece. I find clear pieces require prime unless you are willing to put on many coats of color.

Regarding using a mask, see the current (latest) issue of Scale Auto Enthusiast. There's an excellent article on making masks for clear plastic 'glass'. Optionally, you can view it online at Look for the yellow Camaro on the cover. The web site may show the previous issue with the 'Big Duece'; that's not the one you want.
HTH...
Mike