The mirror film must be thin, light weight and must be applied very smoothly onto the mirror structure surface. Bumps in the mirror surface or a dull reflective surface will decrease the amount of light energy that reaches the pipe. When choosing a mirror film, durability must also be considered. It is possible to install a thin plexiglass surface over the mirror film to increase durability but this will increase weight and can reduce the reflective efficiency.
Please see my test result with different mirror films using my mirror test unit.
My early work was done using aluminium foil attached to 1/4 inch plywood using white glue. My highest efficiencies from sun to oil heating was around 50%. I still have a test unit that is over 1 year old that has been outside and has an almost perfect aluminium foil/white glue/plywood mirror.
The pictures below are of poorly done aluminium foil with white glue on 1/4 inch plywood. With these early test models mirror fillms you can see the bumps in the wood through the mirror film and bubbles in the aluminium foil.
More testing is still required to find the mirror film that will be used for the solar thermal car