Wash them properly and don’t use them for unintended purposes.
After being used to brush on an egg wash or melted butter, a pastry brush becomes gummy and rancid unless you wash it thoroughly before putting it away.
Wet it with hot water and work up a lather by swishing it around on a cake of soap, as if lathering up a shaving brush. Then work the lather well into the bristles against the palm of your hand. Or else plunge it up and down in a container of hot water and dishwashing liquid.
In either case, rinse it well with hot water and air-dry it thoroughly before putting it away in the drawer.
Regarding the damage: Don’t confuse a pastry brush with a basting brush, as several articles in popular food magazines have done. They’re two separate tools, designed to do different jobs.
Pastry brushes aren’t made to withstand heat; their soft, natural boar bristles can melt if used to apply oil or sauce to hot foods in the oven or on the grill. The longer-handled basting brushes, on the other hand, with their stiffer, synthetic bristles, can take the heat without melting.
Just as a pastry brush shouldn’t be used for basting, a basting brush is too stiff for use on delicate pastry.
The cheap paintbrushes with unfinished wooden handles and natural white bristles that are sold in hardware stores are virtually identical to the expensive pastry brushes sold in kitchen supply stores.