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What are some pros and cons to both and what are some decent brands to use for both?

Brush-on:
Pro: No overspray to guard against. No aerosol stink or VOCs. Model shape unlikely to block varnish reaching crevices.
Con: More effort to apply. Runs risk of leaving brush strokes if applied too thick or disturbed while curing. Generally leaves thicker layers than spray and is more prone to pooling in recesses.
Generally trusted brands: Liquitex (which I wouldn't personally recommend, but it's ubiquitous), Golden, Vallejo, Future/Klear (for gloss)

Spray (aerosol):
Pro: Easier to achieve thin, even coverage. No risk of brush strokes or agitation disturbing curing sections. Faster to apply, especially when working in batches or on large models, like vehicles.
Con: Comparatively messy, stinky, and contains irritants. Can be difficult for spray to reach all areas, depending on model shape.
Generally trusted brands: Krylon (I like their Crystal Clear gloss spray well enough), RustOleum (only used an extra thick gloss - wasn't impressed), Testors (Dulcote is my personal favorite when it comes to matte varnish)

You'll note that I didn't mention the dreaded "frosting" as an aerosol con - that was quite intentional. While most people that experience it were using aerosols, it can also happen with brush-on products that are improperly applied (brushed around too much, left to pool, etc.). The only time I've ever personally encountered frosting was when playing around with Liquitex matte varnish, and I managed to get the same issue with both my airbrush and traditional brush application.

Speaking of airbrushes, for the sake of completeness (no idea if you have or even plan to get one), they present an attractive middle ground when it comes to varnishing. They offer a more controllable spray pattern than aerosol cans and don't necessitate the presence of VOCs (some products still require them for thinning/cleaning). They also allow you more flexibility in application method, in that you load them with liquids that could also be brushed on - buy one product and you can apply it either way. Rather handy when you want brush-on products around, anyway, for touch-ups or selective spot applications (like adding gloss to gems and lenses on an otherwise matte figure). Since getting an airbrush up and running, I find myself using spray cans less and less, whether for priming or varnishing. The start-up cost and learning curve is prohibitive enough for most that it's not worthwhile just for varnishing, but an airbrush can be a wonderful tool for the task.