Seeking straight answer re caulking bright decks

[Mark in Modesto]

I am in a bit of a quandary about finishing the mahogany foredeck of my miniscule Wolverine Wagemaker. I've previously posted the question and been referred to the archives. I searched the archives and read way too many posts and still have no answer to my question. Is there someone out there who can just advise me directly so I can get on with this project before the winter fog sets in? Here's the question (it's not that tough!):

Do I varnish the brightwork before caulking, after caulking, or both?

The wood is stripped, sanded, nice and dry and refastened. I intend to use a reddish paste filler and a reddish colored seam filler (caulk)...probably Sika's Chris Craft red polywhatever and the best clear coat I can buy (it's a very small deck).

Any advice, information or input will be greatly appreciated.

Thank you,

Mark Spencer
Modesto, CA

[On Vacation]

Mark, I think people give many opinions but I feel it is totally impossible for anyone to say to you this is that way you should do it with varibles unknown. Heck most of the time a person in your shoes will say "that guy is crazy. He knows that this will not work on my boat" You know the expression about opinions don't you? Well that is all that any of us here can do. You are the actual hands on person doing the job.

[Dan-Q]

Where are you caulking??
Between planks or hardware?

[On Vacation]

Whatever caulking you use, somewhere down the road you have to deal with revarnishing. Make sure the caulking that you are using, whether you caulk before or after varnishing, it will be capable to receive varnish without cracking or able to maintain some height to be taped.

Most people are using colored epoxy now and varnishing over the whole thing. Is the wood pieces laminated to a solid backing? If so, this is the way to do this job. If not consider not using sikaflex. That is my opinion for what you ahve given us. Now I will wait for the "crazy" to start.

[Scott Rosen]

You caulk and pay the seams first. Then your final sanding. Then filling/staining and varnish. Make sure you choose a seam compound that is sandable and that will take varnish. Polysulfide is ideal, but I've only seen it in black and white. The seam compound is supposed to stick to the wood on the sides of the seams to keep the water out. You can see the problem of varnishing first--no wood-to-compound bond.

Good luck.

[ 09-21-2002, 02:05 PM: Message edited by: Scott Rosen ]

[Mark in Modesto]

Oyster, Dan-Q and Scott,

Thanks for asking the questions I shoulda supplied the answers to in my original post. I'll do that now: I am caulking between thin (5/16") mahogany, rabbeted edged (one up, one down) planks, over a wood frame. The undersides are not finished (never were) and the caulk will have to absorb the expansion/contraction our winter fog will create in the planks.

Scott's answer was "spot on" as he assumed correctly the construction of my boat which would, of course, be the crux of the matter. (Darn...wish I'd thoughta that...).

And Oyster...yep, I'm quite familiar with the "opinion" expression...it's one of my favorites, just like the one about "assuming" (see above paragraph), although in this case it worked out fine.

Thanks guys...I now have the dope I needed and more!

All the best,

Mark in Modesto

[thechemist]

Well, yes to all of that.....but

It is likely that one-part caulkings-in-a-tube that have some adhesion-promoting mechanism [meaning they stick and stay stuck] will stick to SANDED varnish down in the seam. If you do any varnishing first put a small radius on the sharp corners of the mahogany, so the film thickness does not go to zero at the sharp corners. Be that as it may, the caulking should be a low-modulus type, so it does not apply great force to tearing the varnish off the wood. That bond, there, will likely be the weak link in the chain.

That said, oil-base varnish over caulking will fail [the film breaks] at the relatively abrupt mechanical transition between the wood and the elastomer, usually. The high-film-tensile-strength clear polyurethanes tend to fail in the middle of the elastomer after a year or two or three, due to ultraviolet ageing reducing film flexibility coupled with the expansion and contraction of the elastomer from cyclic solar heating.

There is not a simple, really good solution to this.

Mahogany expands maybe three to six percent depending on angle to the grain. If screwed down to ribs which expand only a tenth of that parallel to the grain, you must allow some percent of plank width between planks to avoid buckling and/or tearing-loose of fasteners. Elastomeric caulking then goes in from above, with cotton or backer-rod below.

[Scott Rosen]

Chemist,

I'd be very suspicious of a varnish-to-wood bond where there was pulling and compressing forces from the compound. In effect, you're using varnish as the primer for the compound. That sounds like a prescription for a leaky deck.

[On Vacation]

Scott, we actually seal mahogany, especially honduras, before caulking because of the properties of the wood itself bonding poorly.

[Scott Rosen]

Oyster, what do you seal it with?

[On Vacation]

In paint applications, we use a good primer, one coat, and in clear cases, either a clear wood sealer or cuprinol, or a thinned epoxy, depending on the application. This will stabilize the woods especially in end grain situations as teak decks or shiplap planking.

We have had 5200 release from bare wood applications if the wood was not sanded thouroughly, also.

[ 09-22-2002, 11:33 AM: Message edited by: oyster ]

[thechemist]

Originally posted by Scott Rosen:
Chemist,

I'd be very suspicious of a varnish-to-wood bond where there was pulling and compressing forces from the compound. In effect, you're using varnish as the primer for the compound. That sounds like a prescription for a leaky deck.I am not saying it is the perfect way to go, but if you varnish the tops of the wood planks then the varnish film inevitably wraps over and goes down into the seams, and some of that varnish will be in the seam and the edge captured under the sealant. From a weathering point of view it is better not to have the place where the varnish ends, exposed. Thus, the sealant is stuck to the varnish. I am not using it to promote adhesion, I am recognizing that it is what the sealant is sticking to, for better or for worse. Other wood-pretreatment products may be far more effective adhesion-promoting primers.

Similarly, the primary purpose of a lawyer may not be to hold open a door, but if that is what the lawyer is doing at the moment, then he is a legal doorstop.

Wood with cuprinol on it will not exhibit great adhesion for varnish, as copper napthenate is a soap which does not promote adhesion. Some epoxy products are more-or-less compatible with wood so treated, and it is possible to get better varnish adhesion. If the cuprinol interferes with epoxy cure [due to the copper complexing with the amines and deactivating them, so they cannot cure the epoxy, then you will have worse adhesion. The individual epoxy manufacturers can tell you whether their product can tolerate a Copper-Napthenate-rich wood surface. Generally, those with the most critical mixing ratio are the least tolerant.

[On Vacation]

I discussed the cuprinol in the case of caulking the seams, not for varnishing over it in response to Scott's question for sealing mahogany joints.