Core questions

[Mike]

I have a few questions. First can the balsa core on a deck be replaced by ?? Luan plywood? I know the glue is not waterproof in this plywood however it should never get wet if I reinstall the deck hardware correctly. Would ACX be better? I have areas of rot that need to be replaced and I believe I read somewhere that the different expansion and contraction rate of plywood vs. balsa core would cause plywood to show. Some of the outside skin I cut open I will be replacing and some I hope to try and re-use. I?d also like to use the Luan to build up the opening of a hatch. It?s a little hard to explain but a hatch leading into the boat also has side of the cockpit that isn?t molded to increase the opening. This means if more than 3 inches or so of water ever get into the cockpit or if the boat heels with water in the cockpit it would pour in. I?d like to increase the height of the side of the cockpit, and decrease the opening to that hatch. The hatch would actually cover that small addition.

My other question is about the molded in non-skid. Does that need to be sanded smooth before applying paint? It?s fairly deep and if I sanded it flush I would probably need to do a lot of fill work. Will a painted on non-skid look good over it or does it need to be sanded flush?

This is on a US Yacht 25.

Mike

[Hirilondë]

A cored deck or hull works kind of like a truss system. That the 2 layers of structural material (the fiberglass reinforced resin) are separated but attached to each other is what makes it so strong, like the chords of a truss separated by the webbing. The material used as core is in itself not so important. It is however usually very light. Any plywood used as coring would not be very light, so you would then be adding weight to the boat. When the weight you are adding is significantly above the water line it will have some other negative effects than just increasing the total weight of the boat.

I have seen non-skid paint used over embossed non-skid surfaces that look quite good. If there are repaired areas that are smooth as well as the embossed pattern then it can look very blotchy. If you are happy with the general condition then you have to make sure you can clean well into the pattern and do a good job scuffing it all up the create a good bond. Scotch Brite can help considerably as it scuffs down into the pattern. A good 2 part epoxy primer can aid in bonding as well.

[Tim]

Does that need to be sanded smooth before applying paint?

Lots of people apply new deck paint over the existing nonskid--in fact, far more people do this than go to the significant effort to sand off the old pattern. While I have on occasion seen an example of this that actually looks semi-OK, typically it doesn't look very good and to me just screams of poor preparation.

For it to have any chance of success, though, the molded pattern must be consistent. Areas that may have been smoothed during the sanding involved with a recore operation or other repair process will have a different appearance than the nonsanded areas of the original pattern. This won't look good at all.

There's no question that removing a molded nonskid pattern is difficult, time-consuming, and nasty. It uses a lot of coarse sanding discs, and creates copious amounts of very fine gelcoat dust.



It's very possible to remove the nonskid without requring any deck fairing afterwards, any unrelated repair work notwithstanding. Sanding, done with skill and experience, need not be synonymous with dished-out, lumpy nightmares. But a powerful tool equipped with coarse paper in the hands of the uninitiated can create far more harm than good, so there's certainly a level of skill involved.

Nonetheless, removing the old pattern is the right thing to do.

First can the balsa core on a deck be replaced by ?? Luan plywood?

Luaun woudn't be my choice for much of anything. That said, plywood is an acceptable core material, though it is heavier than most core materials. Plywood isn't the ideal material in any event, but if the proper material and installation is used, it will work out OK. The amount of weight added in a boat like yours, while not ideal, is probably effectively negligible in terms of a noticeable effect.

The success of any core depends, in the end, on the quality and stability of the core material. Luaun is generally of pretty marginal quality, with interior laminations made from low-grade material, and often pressed mishmash wood product-like material. While your core might remain dry, as any core material should if installed properly, what happens if the plywood starts to fall apart on its own?

I think choosing a void-free plywood with multiple laminations would be your best bet it you want to use plywood. The glue holding the plywood together is less important than the quality of the inner laminations. This, in the end, is the major difference between plywoods, including "marine" plywood, that meet certain standards, and the typical plywood from the lumber yard. Exterior grade plywood typically is full of interior voids that could affect its performance over time. Don't underestimate the importance of a sound material of consistent density inside the structure of a cored deck.

I would not do it again, but I used marine plywood as a recore material on my boat in 1999. I've frequently discussed my reasons why--namely that the PO had already recored half the boat with this material, and there was enough remaining on hand to complete the job. While I've not experienced any problems as a result, it would certainly not be my choice again in the future. One never stops learning. But even if I'd had a free supply, I never would have chosen a low-grade plywood for the job.

[feetup]

I have a U.S. 25 too.
When you talk about the hatch are you referring to the portside of the cockpit? If so, there are two ways to deal with it. One is to apply some premium weather stripping so that the cover will seal, the other is to either raise the lip with a fiberglass pannel or to build a box up of 1/4 inch plywood and glass it into place, glassing over both inside and out, effectively making a compartment that is not connected to the interior of the hull. I never did like this arrangement of a vast opening into the hull from the cockpit. Are the hinges on your cover made of gold colored plastic? Mine were.(Shudder)
One thing I found effective for small areas of core rot around holes and ports and such was to clean out the rotted core by enlarging the holes a bit and using a variety of bent wires and low pressure compressed air etc. removing the core back to good wood. A wire bent at a right angle in a variable speed drill works well to powder the rotten core through a fairly small hole. Then I filled the cavity about half full of Gorilla glue or equivalent polyurethane glue and closed off the hole with some good masking tape. Disposable hypodermic type syringes work really well for this. The glue will react with any residual moisture in the core and turn to a fairly dense, very sticky closed cell urethane foam which hardens into a great core material that is bonded well to both inside and outside laminations. Fairly large areas can be done this way with multiple holes. I have seen this technique used but with epoxy, which doesnt expand to fill, is quite heavy, and without any fiber reinforsement tends to be a bit brittle. You can use the same technique around openings for portlights, windows etc. by using tape over the exposed edge and filling the gap through screw holes, or even purpose drilled holes above the cavity. The more you cause the expanding glue to come under pressure the more dense and hard the new core will be. Even though the cured glue is waterproof I would still coat exposed edges like window openings with epoxy the same as you would do with any core exposure.
Keep us informed of your progress.

Feetup