Surely the signwriter would point out any such errors . . . .
What? Do you mean because a signwriter would always be adequately attentive to spot their own signwriter's errors, then all signwriters would, by extrapolation, spot all signwriters' errors?
And if there was a trade association for Signwriters with a code of good practice for its members, then it would probably produce Guidance for the Association of Signwriters's members.
I'm reminded of a series of large city centre banners boasting about a major tourist attraction but which ommited a very important letter from the name of the attraction. Apparently no one had noticed. Not the signwriters, not the commissioning agent, not the client, not the riggers of the sign, and not the attraction. After a few days I contacted the attraction which had been so wrongly identified, but no one would believe me - they preferred to politely suggest reasons why I could have been mistaken.
I sent them a photo.
The banners were removed.
Anyway. I don't believe that a grammar check is within the role of a signwriter. That's a job for a pedant! How many professional pedants are there currently employed in the UK?