Apply a thinned coat of varnish followed by at least 2 more coats at full strength to all exposed surfaces including top, bottom and sides of the door. There are also a number of very good water base varnishes, such as Flecto's Diamond Elite line, for outside use. These seem to work very nicely and should have better breathability, flexure and durability than the alkyd products. Here in Miami, we have a lot of problems with the bottom of doors. Because of their location, the bottom of a door is the area exposed to the most weather - sun and rain.
Moisture gets into the surface and under the varnish where it begins to create problems.
These include discoloration, peeling, and cracking. Thank you, Kevin Dear Kevin, Washing out brushes is always a messy bother. There are no automatic washers like there are for roller covers, unfortunately. Various types of brush combs with sharp metal teeth to open up the hilt and allow cleaning fluids into the heel of the brush. Various brush restorers and hardened paint disolvers to soften dried paint and make old brushes usable again.