You may laminate certificates after any required signatures are made.
For pilot certificates, we find this in FAA Order 8900.1:
5-196 PERSONAL POSSESSION OF PILOT CERTIFICATES. Title 14 of the Code of Federal Regulations (14 CFR) part 61, § 61.3 ...
NOTE: Airmen may either use clear laminating sheets to protect permanent FAA-issued certificates or have the certificates professionally laminated, as long as the airman’s signature is placed on the certificate before lamination. Without the signature, the certificate is not valid.
The FAA also allows lamination of aircraft airworthiness certificates. From FAA Order 8130.2J:
When printing a (airworthiness) certificate using the PDF version of the form, set the page scaling to none, or actual size. The printed form should be approximately 5��”by 3-3/8”. Use white 8½” by 11” paper (if the applicant wishes, they may trim the document to size) or appropriate size card stock. You should use a heavy weight paper or cardstock. The forms will print in grayscale; using a color printer is not required. Laser print may be subject to displacement. Encourage applicants to protect the document by lamination or other document protection processes.