If we were to somehow magically recover a Voyager or Pioneer, would it look more or less “pristine” – new and clean as the day it was launched or could you tell its got some miles on it.
Would the foil or other metals be “tarnished” or anyhing from solar wind or the odd Hydrogen atom?
Don’t know how the solar wind would “tarnish” anything, but I’d say that space debris, man-made and extra-terrestrial (micro-meteorites etc) would be the biggest causes of “weathering” and outright malfunction over time.
Probably looks like a sand blaster went at it from all the dust, micrometeorites and solarwind particles.
NASA launched a research satellite called the Long Duration Exposure Facility to study such things; it was deployed in 1984 by the Challenger, with the idea being that it would later be recovered. The loss of the Challenger meant that the LDEF wound up being in space for considerably longer than was originally planned; it was recovered in 1990 (by the Columbia, as a matter of fact 
).
That website I linked to discussed what happened to the various components of the LDEF. As for metals specifically, the answer would be “it depends”:
Some of the metal foils would still be pristine for a long time–gold and platinum are “nonreactive” and should last for awhile out there, while silver or osmium are “rapidly eroded”, copper forms an “oxide coating…that impedes further oxidation” and “may be used for extended periods of time in aplications where thermal management and optical performance requirements are not critical”, and so on.
Of course, that was only in Low Earth Orbit; the effects of the solar wind would apparently have been greater on a probe in deep space. Various sections of that website discuss different aspects of the space environment and different materials carried or used on the LDEF.
The O, Y, and A in the probe’s name would presumably be obscured by space dust.
Um, how does something oxidize in a vacuum ?
Era One: Stars
1977 A.D. Voyager 2 is launched.
16,000 A.D., speed 8 miles per second. Voyager reaches the end of the Oort Cloud.
40,000 AD. Voyager passes within 1.65 light years of red dwarf Ross 248.
5 Billion A.D. Millions of microscopic craters pepper its metal frame. A ragged hole, teeming with stars, gapes in its high gain antenna. Otherwise, Voyager is intact.
8 Billion A.D. Voyager has orbited the galaxy 12 times.
20 Billion A.D. The sun is a shadowy, cystalline sphere of superthick carbon. Earth is dark, dead, frozen.
100 Billion A.D. The still shining stars in all galaxies are almost all dim red dwarfs. The universe has expanded to twice its present size.
1 Trillion A.D. Almost all stars are out.
100 trillion A.D. All stars are out. Black holes, nonluminous white dwarfs, planets, gas and dust remain. The first era of the mature universe has ended.
Era Two: The Liberation of the Planets.
The dead stars and their solar systems move at an average of 30 miles per second. Every 35 light years, on average, two stars pass close enough to perturb each other through gravity. Aproximately 100 close encounters are required to shatter a solar system entirely. Planets are ejected from their orbits or destroyed through collisions.
At some point between 1000 trillion and 100,000 trillion A.D. the second great era draws to a close. Now every world is unbound. Like their decayed parent suns, they steal along private lonely trails through galaxies cold and dark.
1 million trillion A.D. Nine tenths of the stars and planets have been ejected from the galaxies. The rest of the matter has been drawn closer to the increasing dense galactic cores.
The galaxies burst into glowing candescence, going quasar once more. Perilously close Voyager skirts to the black hole’s domain. Deak ahead–a straggling neutron star, itself already doomed. White light from the blazing necklace of the black hole dances off Voyager’s interstellar record, messages that will now never be played. Less than half the original spacecraft remains intact.
And now Voyager is upon the neutron star, banking sharply around its precipitate gravity field. Again the slingshot effect. So Voyager departs the galaxy from which it was forged, and travels into the greater void.
Era Three: Galactic Evaporation
Of the original thirty members of the Local Group of galaxies, five black holes remain, each 10 billion solar masses and a light day across.
Between 1 million trillion (10 to the 18th) and 1000 trillion trillion (10 to the 27th) A.D. all the galaxies in space have reduced themselves by cannabilism to solitary multigalactic black holes. The sole remaining member of the Local Group is a black hole a light week across.
Space is at almost absolute zero. A million light years to Voyagers starboard, a black hole glows at 1 degree above absolute zero. The protons are starting to decay. The black holes begin to evaporate by quantum tunneling.
Fourth Era: The Protons Vanish
Spring in the year 1000 million trillion trillion A.D. Supermassive black holes are the only islands, unutterably small and lonely, in a cosmos awash with electrons, positrons, photons and neutrinos. All “heavy” matter has eroded. The universe has enlarged 10,000 million trillion times since the age of man. The average distance between each electron and positron is greater than the diameter of the old Milky Way.
10 to the 32nd A.D. The electrons and positrons remain too warm to orbit each other. When they meet, they vanish in a flash of gamma rays.
10 to the 66th A.D. Single star sized black holes have evaporated.
100 billion trillion trillion trillion trillion trillion A.D. A positron that came from Voyager finds a mate. The orbit of the two particles around each other is as wide as the entire twentieth century universe. They begin spiralling in towards each other.
10 to the 108th A.D. The last of the black holes evaporates.
10 to the 116th A.D. The majority of the positrons and electrons in the universe have annihilated each other.
A date unknown: The denudation of the last black hole belongs to the forgotten past. Even the final decay of the positronium sea took place lost eons ago. Now there is only blackness, ultimate cold, space without end.
And a positron, the positron from Voyager. In company with some other electrons and their antiparticles, it survived the breakup of the positronium sea. And now these last specks of matter, together with the greater legions of photons and neutrinos, are all that remains of the substance of the universe.
Fifth Era: A single, tiny crumb flakes from the first Hostess Twinkie ever made.
Low Earth Orbit isn’t a vacuum - there’s very low density atomic oxygen, for one. That varies by altitude of course.
Another factor is radiation. One of the nice things the atmosphere does is filter out some of the radiation, so stuff in orbit gets the full dose.
Then there’s temperature cycling. The sun side of the satellite can be over 100 degrees warmer than the dark side.
Most of the degradation would probably be caused by atomic oxygen and radiation, but we don’t usually get a chance to check them, so it’s hard to say.
One of the Apollo flights did recover parts of a moon explorer (Ranger?) to bring back for study, I don’t recall of ever knowing what the results were.