I am required to mail form 8453 and attach form 8283 with appraisal because I donated a piano valued over $5,000. Must I have appraiser and donee sign the form 8283?

Yes, otherwise the IRS will not allow your deduction for the donation of your piano. 

For more information on charity deductions and form 8283 instructions, follow these links:

IRS on Charities and Non-Profits

Charity Contributions

Form 8283 Instructions