I can't tell you what the judge will say about "the angle in which you exited the gas station the sign was barely visible". What I can tell you is that just because you did not see the sign does not necessarily mean that it is not properly posted.
21461. (a) It is unlawful for a driver of a vehicle to fail to obey a sign or signal defined as regulatory in the federal Manual on Uniform Traffic Control Devices, or a Department of Transportation approved supplement to that manual of a regulatory nature erected or maintained to enhance traffic safety and operations or to indicate and carry out the provisions of this code or a local traffic ordinance or resolution adopted pursuant to a local traffic ordinance, or to fail to obey a device erected or maintained by lawful authority of a public body or official.
You can look through the federal MUTCD and the California Supplement and if you find that it does not meet the requirements therein then you might have a decent argument.
Also keep in mind that your contesting the underlying violation of VC21461.a does not relieve you of the charge of Failing To Appear in court for the original citation. In other words, the $512 fine that you mentioned includes a fine for 21461.a (approximately $212) as well as a fine for Failing to appear (approximately $300). So even if you get a dismissal of the 21461.a charge, you may still be held to having to pay the $300 fine for the FTA...