Whether you’re swapping out an old fishfinder transducer, changing a through-hull accessory, or just getting an old boat ready for some new life on the water, it may be necessary to plug old screw holes that lie beneath the waterline. Naturally, any time you’re working on something below the waterline you need to be careful—you certainly don’t want to allow water intrusion, nor do you want to expose any coring in the hull or transom to water. Fortunately, it’s a pretty easy job—tune in for this short Boating Tips video, to see how it’s done.


Remember, using 3M 5200 is the key. This sticky goo cures into a hard rubbery material that’s water-tight and super-strong. And be sure to use both the screw-dip and the squeeze methods of getting 5200 into the holes. You want to be 100-percent sure every space not occupied by the screw itself is filled with 5200.

The best case scenario is, of course, to avoid having to plug those holes in the first place, which could have been accomplished in this case by using the right screws for the job. As mentioned in the video, manufacturer-supplied hardware often isn’t quite up to snuff. Even if it’s stainless-steel it may not be 316L grade (the very best, when it comes to resisting corrosion,) so the safest move is to always source and supply the hardware yourself.

Why not try fiberglassing over the holes on your own? It’s quite difficult to do a perfect patch job this way, and when it’s below the waterline, only perfect will do. It’s not at all like doing simple backyard fiberglass repair, because the constant exposure to water is likely to result in delamination if the glass work isn’t just so. Leave this job to the professionals, period. But just plugging a screw hole or two? That can be handled quickly and easily, right at home.

Written by: Lenny Rudow
With over two decades of experience in marine journalism, Lenny Rudow has contributed to publications including YachtWorld, boats.com, Boating Magazine, Marlin Magazine, Boating World, Saltwater Sportsman, Texas Fish & Game, and many others. Lenny is a graduate of the Westlawn School of Yacht Design, and he has won numerous BWI and OWAA writing awards.