A Critical Observation About Caulking
Posted: 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.
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.