Bedding/sealing compound help

First boat - a 10' lapstrake skiff.
I've sampled 5200 and a locally available polyurethane called Lexel.

The 5200 takes a week to cure and seems a bit too inflexible for laps - maybe great elsewhere. No, I don't plan on using anything at the laps but I want a backup plan if they're not fit tightly enough. (First boat, remember.)
The Lexel is cheap, dries quite flexible and fairly quickly, and is rated for underwater.

Any objections to using one material for betting and sealing such as the above?

Thanks much,
Ed

 

All depends on the type of construction method you are using. It sounds like a traditional lapstrake build. If this is the case and using solid wood for the planking, nothing goes in the seams, just good close fits are required to make the system work. If plywood planking is being used, you'll want to use polysulfide (3M 101, or similar) rather then polyurethane (3M 5200, or similar) because polyurethane will release from soaked wood, where you'll have a better go of it with polysulfide.

If your 10' foot boat will spend the vast majority of the time living in your car port, rather then on a mooring, then 5200 can work for you. Just don't let the wood get completely saturated, by keeping it well painted and repaired and out of the water when not in use.

Lapstrake construction (traditional) requires a high level of beveling skill to produce a water tight hull. For this reason it's not well suited to repairs or first builds by amateurs. Once you get the hang of picking up bevels and plane use on the edges of ply or solid planking stock, it becomes reasonably easy, but the learning curve is steeper then other building methods. If you can't get the tight fits necessary on the laps, with a traditional type of build, then no sealant will come to the rescue for very long. They may work for a short time, but normal plank movement in use and irregular fits will "saw" right through the sealant.

As far as the other polyurethane you mentioned, I've never heard of it, which doesn't mean it's a bad product. My question would be, does it say marine use for below the waterline? If not, don't even think about it. Secondly, you generally get what you pay for, so if there is a substantial difference in price, then the likely reason is it's an inferior product to 3M 5200.

 

Par, Based on what you just said about 5200 releasing from soaked wood, I recently replaced some old non-functioning thru-hull fitting through a cedar planked hull with a taped cedar plug and 5200 to hold it permanently in place. The boat's not yet back in the water. Do I have to worry that these plugs may come out? Scott