mahogany deck caulking

[MB]

Im stripping and re-caulking a 1950 mahogany deck, and have a couple questions regarding this project:

1) original caulking is white...but all i find in the shops here is black sikaflex for the purpose...white (sikaflex 291) is meant for other applications (ie. around windows, hardware etc)...can i use this for deck seams too?

2) just to confirm, that deck caulking is before varnish goes on?....is it ok to put 10 coats over the seams...currently the varnish is cracking over the seams? or should i caulk last to avoid the cracking...

3) if i varnish (Hempel Classic) over white caulking, will it turn yellow.....

If the white caulking discolors etc, then ill switch to black (compromising history for practicality) but stage of when to caulk before or after varnish and coats the seam can handle if after are really the points i havent been able to find info on (since my deck is not teak).

Thanks in advance for advice! MB

[Todd Bradshaw]

If you varnish over anything white it will almost always eventually turn yellow with time. Any slight variations in varnish thickness can also produce uneven yellowing, which just makes matters worse (don't ask me how I know this...) Since calk is normally much more flexible than cured varnish, it wouldn't surprise me to see the varnish crack with age if you varnish over calk. For a durable white calk, I've been impressed with 3M 4000UV Polyether Adhesive Sealant. It's pretty tough stuff. I even used it to calk the joints between the panels of the shower stall in our house and it stands up well to scrubbing and cleaning chemicals.

[pcford]

I assume you are doing some variety of Chris-Craft or a similar boat. If so:

1. Apply varnish 'til there are only a couple coats more required.

2. Mask off with low tack tape and in live seams use Sikaflex 240 (I think that is the number. ?)

3. Mask off with low tack tape and in fake (sawkerf) seems use quart glazier's putty which has been mixed with one tablespoon varnish, kneaded to mix.

4. Let caulk set a few days.

5. Varnish a couple more coats.

6. Mask off with low tack tape and paint seams white. Some prefer a pale yellow color to mimic the color of varnished seams. It is a mellow effect.

[islandteak]

There is a new Sikaflex 290 DC that is specially designed as a deck caulking with greater ability to adhere and withstand movement, UV etc.

...Ken

[Roger Cumming]

1. Prepare the wood deck for varnish. 2. Apply polysulfide sealant such as Boatlife, color of your choice. Use masking tape to get perfect seams. 3. Varnish 8 to 10 coats over everything. 4. You're done.

[pcford]

1. Prepare the wood deck for varnish. 2. Apply polysulfide sealant such as Boatlife, color of your choice. Use masking tape to get perfect seams. 3. Varnish 8 to 10 coats over everything. 4. You're done. I prefer the method I describe in the previous post because....

1. Wood should be varnished and sealed so that there is no chance of caulk going into bare wood. Or maybe I'm just sloppier.

2. Boatlife breaks loose from sides of seams. Does not stick well in this application.

3. It's worth mentioning again than you should use low tack tape for masking. (the white papery stuff.) I've had varnish pull up down to bare wood when using Fineline tape. (the vinyl beigey stuff)
If that happens, it challenges the depth of my curse word vocabulary.

[JormaS]

My vote goes to Roger´s method, because it is straight forward and simple.

Eventually, the varnish will become hard and it will there fore crack and/or crumble over the seams because there will be some movement. However, that may happen after a very long time because the deck planks are probably quite narrow and they will be covered with varnish, and thus the movement will be small.

When that happens, I believe the only sensible thing to do is to remove all varnish and start from scratch.

What caulk you put in the seams is not that important because of the minimal movement of the wood. With wide and unprotected planks it is different.

[pcford]

My vote goes to Roger´s method, because it is straight forward and simple.
Eventually, the varnish will become hard and it will there fore crack and/or crumble over the seams because there will be some movement. However, that may happen after a very long time because the deck planks are probably quite narrow and they will be covered with varnish, and thus the movement will be small.

When that happens, I believe the only sensible thing to do is to remove all varnish and start from scratch.

What caulk you put in the seams is not that important because of the minimal movement of the wood. With wide and unprotected planks it is different. The only mahogany decks with which I am familiar are on runabouts or small cruisers. I assume this is the case here.

If that is the case, then the apparent deck planks are most likely three times wider than they appear to be. That is because the apparent approximately 2 inch wide deck plank is actually part of a 6 inch plank. The reason for that is that only every third seam is an actual live seam. The ones between are merely saw kerfs. Thus, the live seam has 6 inch planks to each side. And there is a fair amount of movement. Thus, unless the caulk sticks well, it will let go. Polysulphide, especially single part like Boatlife will let go. I use glazier's putty on the intermediate (saw kerf)seams because it's cheap, easy to use and clean up is easy.

I hope I've explained this well. This is pretty much standard procedure in a professional runabout restoration shop. I've made a living doing this stuff for 25 years. I ain't guessing.

[ronchatfield]

I am building an 18' mahogany runabout replica and am very interested in the topic of deck calking as I am close to applying the mahogany decking. I am thinking of making my individual deck strips 2.5 - 3.0 " wide, not 6" w/kerf cuts; (is this about the right width?) I am closing in on 4 years construction time for this boat and don't want to rush things at this point but am eager to learn of ways to reduce work effort and time without sacrificing work quality. At some point I will be applying 15-20 coats of varnish to the deck but don't want to "yellow" the calk from varnish. What do you recommend? From the responses I've seen so far, there are several choices. However, I don't want to spend several days taping off the seams if there is a viable alternative.

Thanks in advance for your adice.

Ron Chatield

[pcford]

I am building an 18' mahogany runabout replica and am very interested in the topic of deck calking as I am close to applying the mahogany decking. I am thinking of making my individual deck strips 2.5 - 3.0 " wide, not 6" w/kerf cuts; (is this about the right width?) I am closing in on 4 years construction time for this boat and don't want to rush things at this point but am eager to learn of ways to reduce work effort and time without sacrificing work quality. At some point I will be applying 15-20 coats of varnish to the deck but don't want to "yellow" the calk from varnish. What do you recommend? From the responses I've seen so far, there are several choices. However, I don't want to spend several days taping off the seams if there is a viable alternative. Traditionally in runabout construction, the live deck seams land on a batten. Obviously, you couldn't have a batten under each plank in your planking scheme you descibe above.

A couple suggestions. You could laminate the separate planks to a piece of plywood. Planking is done this way in southern California where extreme environmental demands require a way of stabilizing the deck. In this case, the planks are not separate, but they use the 3 apparent planks in one real plank scheme.

Another method that I have seen is to have the seams actually caulked. This was done in very old boats. I have seen only one example of this style of construction...a 36' 1929 Hutchinson runabout.
In smaller boats a caulked deck would not be practical. Deck are usually 3/8". Too thin for normal carvel style caulking.

All in all, I don't see much advantage to separate planks. It would control the working of the seams but at the cost of increased construction difficulties. Not worth it, in my humble opinion.

Apropos traditional deck planking: always have a plank with VG or slash grain. If flat grain, the pattern will cross the apparent planks and spoil the illusion.

Oh, taping seams is tedious but will take only a few hours. Do it when you put caulking putty in and when you paint. If you use glazier's putty for the fake seems, you don't have to mask them off. It cleans up nicely.

Regarding yellowing: paint the seams. Case closed.

I do this stuff every day to make a buck and have for about twenty five years. What I have outlined is commonly accepted practice in professional runabout restoration shops.

I'd certainly recommend Mr. Dannenberg's book for these details of runabout restoration.