Your initial reaction to having a dedicated brush that you use only for varnishing a painting may be that it's an unnecessary extravagance. Why not just use one of your larger paint brushes? Well, considering that varnishing is one of the final things you do to a painting, and probably only to those paintings you think worthwhile, isn't it worth a small investment to ensure it's done properly? A varnishing brush isn't going to wear out in a hurry, so you won't have to replace it very often. A good varnishing brush helps ensure you get a smooth coat of varnish. And by using it only for varnish, it will never get tainted by paint.
You're looking for a flat brush which is at least a couple of inches (five centimeters) wide, about a third of an inch (1cm) thick, and has got long hairs.These can be either synthetic or natural hair, but either way should be soft with a bit of spring.
You don't want a 'scratchy' brush that will leave brush marks in the varnish. Check that the hairs are well anchored, that they're not going to keep falling out as you apply the varnish.
Larger art material stores and online-art stores should stock a range of varnishing brushes. Pick them up and see how comfortable they feel in your hand. Alternatively, look in your local hardware store -- though you may want to cut off some of the hairs to reduce the thickness of the brush, and be sure to avoid cheap DIY brushes whose hairs will almost certainly fall out regularly.
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