What would cause a white film on the assembly?[causing lockout].

Properties in the environmental air around your furnace. Hair spray,dryer sheets,laundry soap,chlorine,are just a few items that when burnt by the pilot will cause the problem you are having.

There is also the likely hood of the gas you are burning can also cause this. Some of the suppliers are adding different chemicals to natural and propane gas to help boost its output to the government requirements. They take a non conforming gas and with the addition of chemicals make it as good as the standards require.

hvac1000 is correct on his answer... as well, burning LP ( propane) seems to cause the oxidation on flame rods faster than Natural gas. You generally can clean flame rods with rough steel wool, or sand paper, which is really not the best choice. Best is to really replace them.