I am doing some link building for a company who sell surveying equipment. If I add their domain in my signature on dofollow sites, will that add benefit to them being that they are unrelated?
yes it will. Make sure you leave valuable comments though. Don't spam. The sig links has helped me to a point - but this isn't the only way.
So for example, if it is an SEO forum and my comments relate to the subject I reply to or post, my irrelevant sig link will still have value.
The dofollow link from the signature to the company website would count for PageRank style calculations. Being that the forum and the company are not related though, the link probably wouldn't bring human clicks often.
In my opinion unrelated links also help. For one of my sites, most of the links are from 'personal' blogs and the site does well in SERPs. Though you will get much better results if you get links from relevant sites.
Unrelated links are fine, but it's worth your time to find related sites. It's going to take the same amount of time to find an unrelated forum as a related.
Definitely, it will help your client. Make sure to use some of those golden keywords and start posting quality posts in forums. DP forum is giving dofollow in signature. I'm not sure any other forum that is related to surveying equipment gives dofollow.