below the water sealer needed

hi
I need advice. I have a 1940's Mastercraft 14 foot plywood rowboat that is in great shape. To protect the orginal paint and help with a few minor leaks I think I should put a top seal coat on it (like a varnish). I have not yet found a clear product that can be used above and below the water line. Any body have any ideas?

 

I'll assume your Mastercraft is plywood over frames. Minor leaks will not be prevented with a clear top coat of any type. Leaks in this construction type are typically from two sources, working fasteners and/or opening seams (probably both).

No coating, baring a thick, reinforced sheathing, which isn't clear, will prevent the movement or close up seams. Only repairs to the issues involved will fix the leaks. Generally, you'd recaulk and refasten a sound structure, which restores it's integrity and water tightness. If the problem has been ignored for a while, more issues crop up fairly quickly too. These include; water intrusion into the frames, floors, stringers, etc. Rot in these areas (and others), "egged out" fastener holes, cracked and broken structural elements and the list can go on and on.

In short, if the boat is kept on a trailer, under cover, dry and well ventilated, then a varnish or polyurethane coating will protect it well enough (assumes good care and upkeep), though it'll still leak. If it lives on a mooring or at a dock, there's no clear coating that can tolerate continuous immersion.

 

Wooden boats often need to swell to stop leaking.

 

PAR has really hit the nail squarely here. There is no clear top coat you can put over old paint that will preserve it, resist moisture and UV and last more than a few weeks. Painted plywood boats that are 70 years old are probably due for a session of burning off the paint and re-coating. Once the plywood is bare and sanded and any structural issues dealt with, an epoxy sealing coat is great, but under the paint, not over. The epoxy will make the new paint job stable and last a long time.

 

... but not a plywood on frame boat,

 

It seems you are trying to protect paint by applying something clear over it, and not specifically asking about fixing leaks at the chines and seams.
Varnish isn't much more water-proof than paint, nor will the coating be clear enough to be invisible. Varnish is usually close to amber in color (though some more clear products might be available).
No, the way to waterproof the hull is to epoxy-coat the bare wood prior to painting, which is a great way to preserve both paint and wood. However, water sitting in the boat will saturate the plywood from the inside so you must completely coat the inside as well.
Maybe better to simply make sure you've got enough layers of paint (two or better, three coats) on the outside of the hull and leave it at that. Sometimes improving the impermeability of the surface isn't less work than upkeep in the form of a coat of paint every third year (depending on usage). In any case, either with or without epoxy, both ways have merit. Epoxy is expensive and is a lot more work up front for less work later, and plain paint is the opposite.
Choose your poison.

 

Plywood swells a bit and the wood it is attached to-stem, keel, etc. swells and takes up too.

 

I don't think epoxy on a plywood over frame build is a good idea. Epoxy is only effective as a wood stabilization process, with total encapsulation. Anything short of this will permit moisture gain and worse limited avenues of escape, which leads to other issues.

Unless the boat is fully dissembled, literally embalming each piece, epoxy probably isn't the best way to go. If it was my boat or restoration, I'd do the usually to a 70 year old boat, check the condition of the planking and structural elements and repair/refasten as required. Refastening will likely require restoring most if not all the fastener holes. Drill them out, epoxy in a dowel, then refasten as necessary, knowing the fasteners will grab threads full of meat, not mush.

The other option is a heavy sheathing, set in epoxy, though this has it's issues too. If the sheathing is heavy enough (12 to 16 ounces on this size boat, done in two fabric layers), it will resit external moisture when underway, stabilize the seams and reinforce the renewed fasteners, so they don't have to work as hard. Of course this is more costly then just fixing the boat with traditional plank on frame methods, but some find the sheathing reassuring.