A Critical Observation About Caulking

Repair or modification of Boston Whaler boats, their engines, trailers, and gear
Thetacker
Posts: 6
Joined: Wed Oct 27, 2021 2:05 pm

A Critical Observation About Caulking

Postby Thetacker » Mon Nov 15, 2021 6:02 pm

I have been reading incessantly, as I have been preparing to re-caulk all the decks seams on gunwales and the fuel-tank hatch cover on my 1999 17 Outrage II. There is certainly a lot of dialogue about techniques and discussions about the best sealants to use. However, I have noted a relative absence of discussion about a critically important topic that will often be determinative of sealant performance: gap ratios.

All sealants are dependent for their performance on having the correct gap (width to depth) ratios and in avoiding three-sided (i.e, gap bottom) adhesion. Most silicone sealants call for 2:1 width to depth. Some other sealants call for 1:1 ratios. No sealant can tolerate being adhered to the bottom of a seam, if optimum performance is to be achieved. Many decks seam (hatch covers, etc.) are far deeper than they are wide, often at 1:4 (i.e., much deeper than wide) ratios or more, or in some situations, the reverse. If sealant is merely poured into such gaps and adheres on all side, adhesion failure is almost certain, as the product elasticity will be lost, as the product becomes a too bulky solid. Being adhered to the bottom of a seam also significantly decreases elasticity. The end result is that sealants suffer adhesion failures, and they separate from one side of the material to which they are attached.

The way to avoid such failures is to be sure to use a backer rod in deep seams, adjusting the depth of such rod, so that the seam has the desired width:depth ratio for the sealant being used and no three-sided adhesion occurs.

There are probably many more sealant failures due to incorrect gap ratios and three-sided adhesion than resulting from any outright failure of the various sealants themselves. I just wanted to mention this critical detail because I noticed so little commentary on this vital topic.

I might add that I discovered my Boston Whaler boat had exactly this problem. Even the factory simply poured sealant into the seams until they filled up, without any regard to gap ratios or what effect that would have on sealant performance. One would think they would know better.

jimh
Posts: 11672
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: A Critical Observation About Caulking

Postby jimh » Tue Nov 16, 2021 8:01 am

Please explain "a backer rod."

Jefecinco
Posts: 1592
Joined: Fri Oct 09, 2015 6:35 pm
Location: Gulf Shores, AL

Re: A Critical Observation About Caulking

Postby Jefecinco » Tue Nov 16, 2021 9:59 am

Backer rods are used to help fill gaps that would otherwise require an inordinate amount of caulk to seal. Backer material can vary depending on the gap size, shape or length. Small diameter dowels can be useful for straight seams. Flexible material is useful for curved seams and for going around corners. Solder and cordage are particularly flexible. Selection of backing material should always consider the backing's compatibility with the caulk and with the material it will contact.
Butch

Thetacker
Posts: 6
Joined: Wed Oct 27, 2021 2:05 pm

Re: A Critical Observation About Caulking

Postby Thetacker » Tue Nov 16, 2021 10:27 am

Backer rod example: https://www.acehardware.com/departments ... ng/5395751

Backer rod is available in various diameters.

jimh
Posts: 11672
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: A Critical Observation About Caulking

Postby jimh » Tue Nov 16, 2021 10:59 am

Thanks for the link to a backer rod. I note the following text which describes the purpose:

The backer rod is recommended for use in filling large gaps and openings to prevent drafts.

Source: the link cited above.

Q1: What difference occurs in the elasticity of the dried caulk when the caulk contacts the backer rod compared to the caulk contacting the bottom surface of the fiberglass boat?

Q2: How does the caulk avoid the "three-side adhesion" problem by contact and adhesion with the backer rod?

Q3: Is the backer rod made of a foam material?

Jefecinco
Posts: 1592
Joined: Fri Oct 09, 2015 6:35 pm
Location: Gulf Shores, AL

Re: A Critical Observation About Caulking

Postby Jefecinco » Fri Nov 19, 2021 10:26 am

The Ace backer material appears to be made of some type of foam. I assume it's available in various diameters. It is undoubtedly intended for home projects or similar. It is likely fairly handy as it appears to be compressible and thus can be squeezed into gaps sealing them before caulk is applied.
Butch

RWM
Posts: 29
Joined: Wed Nov 11, 2015 6:24 pm

Re: A Critical Observation About Caulking

Postby RWM » Fri Nov 19, 2021 1:32 pm

About a year ago I replaced the deck caulk on my 1986 18’ Outrage. After a thorough prep I used a couple sizes of Ace Hardware foam backer and GE Silicone II Almond.

[The new caulk] looks good and is holding up well. Used only one tube for the whole deck, and there was a little left over.