Light interacts with fresh metal surfaces in only the first few atomic layers. What makes metals "metals" is the very high electron density, and we can think of that electron "plasma" as having such a high plasma frequency that the light barely penetrates a tiny fraction of a wavelength before being re-radiated backwards by all those electrons vibrating along with the incident electric field.
See Wikipedia's Skin effect. In the plot below we can see that even at a radio frequency of 1 MHz an electromagnetic wave's fields will have dropped by 1/e in only 10 microns when incident on a smooth, polished surface of 304 stainless steel (we can assume 301 to be similar). It drops as $1/ \sqrt{f}$ so for red light of 600 nm or 5E+14 Hz that line would reach about 1 Angstrom. We can't do that because we have to take microscopic plasma density effects and other goodies into consideration, but it works out about right. If we wanted a more accurate answer we'd have to look up the complex index of refraction $n + ik$ and then calculate the attenuation coefficient.
But I digress
because the surface of a rocket ship is not an atomically smooth polished surface even before launch. Even for stainless steel will be some contaminants adsorbed on the surface and some of the impurities will be oxidized, those "nooks and crannies" of realistic surfaces will have wavelength-dependent scattering effects.
Put it in deep space for 50 years and the effects of micrometeorites will modify the surface and that of ultraviolet light and the continuous onslaught of charged particles and neutrals from the Sun will have implanted and modified the top several tens of nanometers of the surface so much as to make the electronic structure and optical response very different than stainless that's remained on Earth.
The operative concept here
is "the top several tens of nanometers" which is pretty much all that the solar wind will effect.
This has no effect on 304 stainless steel's structural properties.
If it were a front-surface telescope mirror or even a dish for a sub-millimeter radio telescope, it would matter optically, but not structurally.

Wikimedia's File Skin_depth_by_Zureks.png