Solargraph Failed due to overexposure.

As mentioned earlier, one pinhole camera disappeared a month ago and yesterday I found out that after 6 months exposure my other one did not show much details.
Bad luck but what can we learn from this?
My conclusion of the latter is that the paper was overexposed. This probably because the can (this time a used a piece of a "sewer pipe") wasn't light prove, so the sun could penetrate through the can and did expose the paper.  Instead of exposing the paper via the pinhole.