Question: I am making something for outdoor use and I've painted it with a matte exterior latex paint. I'm satisfied with the protection that the paint will offer but I'm experimenting with ways to make it look nicer.

Right now I am considering glossy finishes. I have found that I much prefer the appearance and texture of shellac (de-waxed) to a high gloss spar urethane.

I don't really need good scratch resistance for this particular piece, and the paint should protect the wood enough from moisture and sunlight.

I'm also not too concerned about color changes over time as long as it is uniform. The paint is just brown, and maintaining the precise brown isn't super important.

My question is, how should I expect a fairly thick shellac finish (over paint) to behave outdoors over time? It will be exposed to sunlight, a wide range of temperatures, and plenty of rain, snow, and ice. Should I expect flaking? Spotting? Blemishes (I've heard it gets cloudy with moisture)? Other weird stuff? Or will it maintain its nice appearance? If it does get grimy, will it be prone to staining (with polyurethane a good wipe down with an all purpose cleaner takes care of it)? 

I already own the paint and don't really want to buy another gallon of glossy paint; I'm trying to do this with the stuff I already have sitting around collecting dust on my shelves.

Answer: 
Shellac doesn't do well with water, that's your starting point here. It's also relatively brittle and with the expected movement in the wood you're likely to see problems with cracking. 

Bonding to the paint shouldn't be an issue initially at least, the perceived wisdom is that "shellac sticks to everything and everything sticks to shellac" and while that's a bit of an exaggeration it should bond very well to just about any paint. But you may see the paint shucking it off after multiple cycles of swelling and contraction.

The main objection of course might be shellac's well-known issue of clouding when exposed to water. 

> 
> I already own the paint and don't really want to buy another gallon of glossy paint; I'm trying to do this with the stuff I already have sitting around collecting dust on my shelves.
> 
> 
> 

I would suggest you try thinned varnish. It's not inherently a bad idea to coat paint with varnish, it's actually done more commonly that we tend to think although this is for interior projects. However, there is a potential problem here in that you'd probably want to use spar varnish (specifically formulated to be more flexible than regular varnishes) but this is noticeably yellow or amber and could give too much of a colour change for your liking, even if applied in 2-3 thin coats.
Shellac is perhaps the worst choice for the outdoor use. If you ever set a wet glass on shellac finished furniture, you'll notice a white ring. Water softens shellac and overtime it will wear away from the surface. When it softens with water, it has a tacky feel which is a magnet for dust. When it dries, any dust or dirt essentially glues to it.

The lowest cost solution, is to just leave the paint as is and reapply it when it looks weathered. If the paint you're using is a good quality latex paint, it won't crack and flake off.

If you want a very durable and relatively inexpensive solution, get some polyurethane (preferably water-based, since it doesn't darken the color) and wipe or brush it on.