Working on experimental aircraft from a mechanic's point of view is touchy, at best. I'll work on someone's aircraft after viewing the aircraft talking to the owner, discussing the availability of relevant data, etc. Howevr, I reserve the right to refuse part or all of the work at any time.
I've seen some downright outstanding workmanship on experimental airplanes, and work that is scary. I've seen people put in very compromised situations merely by trying to help someone out. The mechanic who signs the logbook or signs for work done, especially for an inspection, "buys" all the work that's been done in the past...and put his head on the block until that work is done again in the future, or until the next inspection. That's asking a lot...especially where the only leg to stand on is trust that the owner is a good guy.
Do you need a mechanic to sign off the work on your airplane? No. If you built it, then you can apply for and receive a repairman certificate, and you can repair or modify it as the aircraft manufacturer. If you have certificated components on board that you wish to retain their value and certification, then these will need to be inspected, repaired, and serviced by qualified and approved individuals, but that's another matter.
Should you seek the help of a certificated mechanic in dealing with your aircraft? You'd be foolish not to...but it's your choice.
14 CFR 43 prescribes the regulations pertaining to maintenance, preventative maintenance, alternations, and rebuilding. This paragraph begins by telling you that it doesn't apply to experimental aircraft, as follows:
§ 43.1(b): This part does not apply to any aircraft for which an experimental airworthiness certificate has been issued, unless a different kind of airworthiness certificate had previously been issued for that aircraft.
Don't be surprised if mechanics are reluctant to work on your aircraft. Mechanics are accustomed to being held to very exacting requirements to ensure all their work conforms to approved data, and to provide the source, and to ensure it's current material. Simply doing work willy nilly goes against the grain of most in the business; we already have a great deal of liability hanging over us, usually for very little compensation. A lot of folks aren't excited about hanging out even further for an unknown quantity. After all, if something happens, what has the mechanic to fall back on for protection, or even to show as a source for data to have accomplished the work performed? Nothing.
I'll work on an experimental airplane, but usually only if I have performed a very thorough inspection, have either personal knowledge or familiarity with the pilot and builder or someone who I do know who will vouch for that person, and acceptable data upon which to go for the work performed. That may include plans used for the aircraft construction, AC 43.13, etc...but I'm not going to simply effect repairs or sign anything without having a very solid leg to stand upon. Don't be at all surprised if most mechanics feel the same. Just doing our job is enough of a hazard and a liability, making it riskier doesn't engender the warm fuzzies.