Originally Posted by Pete7
Yep then cover bolts and washers in a big dollop of sealant, though I do accept lining the holes with a layer of resin would be good practise.
There's 2 avenues of problems caused by the drilling of the bolt holes: 1) You want to keep water out of the structural part of the transom, hence sealing the bore with epoxy, glas resin, or similar. 2) you want to seal around the bolt, so water doesn't weep through to wherever the hole ends up on the inside, and to prevent anaerobic bacterial corrosion from eating away at the bolts (I'll explain that one below, in case you're not familiar with it), hence the sealing of the bolts into the holes.
Most likely, taking care of one problem will take care of both, for a while. If, for whatever reason, the bolts are removed (or even just loosened and retightened), you open both avenues back up again. Most people will not go through the trouble of re-sealing the holes again, and the problems are likely to start there. By taking care of each in turn, there's less likelihood that both will start happening.
The anaerobic bacterial corrosion happens in tight locations, where water gets in and doesn't dry out. In essence, the existing bacteria is active as long as oxygen exists in the water, then dies off as it's consumed. The anaerobic bacteria then begins feeding on the ex-aerobic bacteria, giving off sulfuric acid as a by-product. The acid eats away whatever it can, including stainless steel. This type of corrosion is pretty common in stainless-to-stainless threads that are submerged. The simplest way to avoid it is to use a non-soluble filler for the threads (i.e. sealant, caulk, etc) which will prevent water ingress in the first place.