Polished metals reflect sunlight very well - up to 90% for a highly =
polished aluminium surface. This property is easily compromised by any =
change within a depth of a few atoms near the surface. Oxidation is such =
a change and takes place quicky on most metal surfaces (except for a few =
'noble' metals whose chemical reactivity is very low). In aluminium, =
this oxidation does not look like rust and may be invisible to a casual =
observer, but causes an immediate loss of reflectivity, down to 40% or =
less. Surface contamination, dust and dirt will reduce it even further.

White paint has a reflectivity of about 20 to 30% in the visible light =
spectrum when well maintained, and is generally preferable as protection =
against solar gain.

There is a reason why a metal surface in the sun "burns the skin" while =
a painted one may not. It isn't because the metal surface is warmer - in =
fact it may be at the very same temperature. Metals, however, are =
excellent heat conductors, and will transfer heat very quickly to other =
bodies with which they come into contact. Paint, fiberglass, wood are =
essentially organic materials held together by covalent bonds and for =
this reason conduct heat less easily.

On the inside of a trailer, the property that matters most regarding =
heat transfer is the surface emissivity. Metals have a lower surface =
emissivity than other materials, even when lightly oxidised. For this =
reason, the innards of a metal trailer may well remain with their =
natural finish, except if the local climate is conducive to water =
condensation in the winter. In the later case, an internal lining with a =
vapour barrier will hinder condensation inside the trailer. Even just =
plain painting will reduce the severity of internal condensation.

Lining the inside of the trailer with any metalised, reflective foam, =
preferably leaving a small gap between this and the trailer walls, will =
do marvels in lowering internal temperatures. This is very cheap =
material and may be considered by anyone keeping a glider in a sunny =
place with large daily temperature swings. A ventilator helps too, =
mostly in avoiding condensation.