This is from the LA Building code:
(c) Design and Construction. Sign frames and sign support structures shall be designed and constructed in conformity with other applicable provisions of this code, and, in addition, shall comply with the following:
1. Each sign and sign support structure shall be designed to resist the wind pressure as required by Section 91.2311 of this code.
2. Structural steel members shall be not less than 1/4-inch thick if ungalvanized, and 3/16-inch thick if such members, including connecting bolts and fastenings, are galvanized or otherwise treated with approved weatherproofing. Bolts used to support signs equal or greater than 100 square feet in area shall not be less than 1⁄2-inch in diameter. Bolts supporting signs less than 100 square feet in area shall not be less than 3/8-inch in diameter.
Signs erected on buildings or structures shall be securely attached by means of adequate metal brackets, expansion bolts, through bolts, or lag s c r e w s . No materials, fabrication, or equipment shall be used which may become dangerous because of vibration, corrosion, disintegration, or any other reason. Wire, other than stranded cable, shall not be used.
If supports of an existing structure are found to be inadequate, they shall be adequately strengthened before the sign is erected.
(d) Materials. Signs and sign support structures may be constructed of any material allowed in this division for the classification of the sign to be installed. Any materials used shall be of the same quality and grade as those specified in this code for use in buildings. In addition, any glass material used in signs shall comply with the limits set forth in Table No. 62-A.
If a coating is going to be used, you might need to find out what Fire Rating it needs to pass. You would be fine if it makes the wood Fire Rated "A" - just document the materials used when submitting for permits.
Personally, I would probably not slap wood against the structure. I would make a frame and attach the wood to that, then hang the structure as a unit... giving the wood and structure a barrier for even more protection.