I picked up a new Garmin depthfinder yesterday. I need to remove my old transponder (from the dead Humminbird) and install the new transponder on the transom. It's a fiberglass hull. Exactly what specific sealant do I use for sealing the screw holes? My local dealer told me to head up to Home Depot but couldn't tell me exactly what to use. The only thing I can think of is 3M tub and tile adhesive caulk, but there has to be something better. Or is there?

theres probably something better  but tub and tile caulk will work fine   ive done it on my ol boat and so have others that i know of and theres never been a problem

Skwerl,

I would use nothing other than 3M 5200. You can find at walmart in the boating section.

http://www3.3m.com/catalog/us/en001/auto_marine_aero/marine/node_GSWWSBSW85be/root_GST1T4S9TCgv/vroot_GS4JK4Y166ge/gvel_QH8HT14PGTgl/theme_us_marine_3_0/command_AbcPageHandler/output_html

3M 4200 cures faster. http://www3.3m.com/catalog/us/en001/auto_marine_aero/marine/node_GS46L96VMDbe/root_GST1T4S9TCgv/vroot_GS4JK4Y166ge/gvel_QH8HT14PGTgl/theme_us_marine_3_0/command_AbcPageHandler/output_html

it is just not as strong.

Paul

3M 5200 is going to be a permanent fix.  You will have a hell of a time removing it if you ever need to.  4200 is a little less permanent.  Also 5200 will require several days to curry fully so don't apply now and fish in the morning.  The best fix if you are not going to reuse the holes would be to mix some epoxy and cabisal(sp) to fill the holes.  Should be available at your local boating center.

Good Luck,

Tom D.

Well I got antsy and installed the new Garmin 250 fishfinder today.

wasn't going to go driving around trying to locate the 5200 so I started digging through my toolbox and found a brand new tube of clear silicone RTV adhesive sealant. Actually it is the same stuff that was used on the previous fishfinder. I used my air compressor with a blower nozzle to blow out the holes good and stuffed them with the RTV. Also squirted it in the new holes and coated the screws before running them in. I think it'll be just fine.

Why does every single manufacturer of fishfinders need to create different size brackets for their transponders? :-/

So we have to put new holes in the boat every time we change them.  The back of my boat looks like it was in a gun fight.  I think there is more silicon back there then aluminium.

Glad you got it done,

TD