The Mooney maintenance manual suggests using a mechanical scraping device fashioned from plexiglass sheet. There is/was someone out there offering a solvent recirculating system (pumped/sprayed solvent in fuel bay until sealant was removed), but I've heard it is murder on the aircraft paint on the lower wing and should only be used if a re-paint is imminent. IIRC, the sealant is actually three sealants, each applied successively (might be two, too lazy to check the manual). Regarding the inspection panels, you will need to remove all of them (top and bottom) in order to access and remove all sealant in the tank. Stripping the sealant is a nasty, labor intensive, minor injury inducing (lacerations on arms and hands) job that any A&P will be glad to let you do; doing so will save you ~10-20 hours labor charges per tank. IIRC, the last time I checked, the sealant required for one tank was ~$800 and takes about another 8 hours per tank to apply. IOW, you can have one tank resealed for ~$1500 if you do the dirty work and then have a good shop apply the sealant and close the tanks (that sequence keeps it all nice and legal) or you can pay $3500/tank to have one of the "name" shops do the work for you. I've done both in the past, but I'd opt for the first (cheaper) answer if I had it to do again. FWIW, the "name" shop was Top Gun, Stockton, CA, and they did a great job, but IME the extran $2k per tank did not net any increased tank seal integrity, sealant longevity, warranty coverage, etc. YMMV; others have had contradictory experiences.
