View Full Version : Caulking, a cautionary tale

William R Roche
03-19-2002, 12:26 PM
If you want to read about what can happen when you caulk with synthetic "caulking material" only, see the following UK Marine Accident Investigation Report: http://www.maib.detr.gov.uk/sd/0301/25.htm

Ed Harrow
03-19-2002, 12:37 PM
Are we to interpret that as saying there was no cotton or oakum used in the seams? Darwin at work here. Somebody was lucky.

Bruce Hooke
03-19-2002, 12:50 PM
Lesson number 2:

"2. Some sealant manufacturers claim that you can use their products without caulking cotton or oakum. It may be possible to achieve good results this way provided the manufacturer's instructions are followed for the use of primers etc. You can't get compression in the seam by just using a synthetic sealant..."

Seems to basically say: we can't say "don't use sealants without caulking cotton or oakum" because it might work under some very special circumstances and besides the manufacturers would jump on us if we said that, but since you can't get good compression without caulking cotton or oakum you draw your own conclusions...LOL

gary porter
03-19-2002, 01:34 PM
I have a friend whos commercial fishing boat planked with yellow cedar is only caulked with polysulfide. Its about 25 years old now and seems to be doing ok. However, this boat is always in the water except to pait the bottom etc. and lives in Ketchican Alaska were its most always wet. So I think there isn't much change going on in the wood. I'm not sure why it was done this way, perhaps it was economical at the time.

03-19-2002, 04:05 PM
Sealing with mastics require a decent 'body' of mastic to do the job - whether that job is stopping a hull or joining plates of glass in a building.

Mastic does not like thin feathery edges.

6mm, or a quarter inch in old money, is about the minimum for a lasting joint and the traditional tapered V joint is not the best design for mastic use.

Better to router a 6mm x 10mm groove, prime the edges and fill with mastic. Priming with the correct primer for the mastic is cruicial to the finished job.

When done properly, the wood parts before the mastic. You can find out what a good job it can do when you have to remove a damaged plank. I have removed all fastenings on a well sprung strake and ended up having to chop it out - no hope of using it as a pattern.

Don't use oil based mastic [ the cheapest ]. One part polysulphide is ok, polyeurethane is better and two part polysulphide the best. No point in saying brand names - they differ so much.

I still like to caulk with oakum on an old boat as it takes a lot more movement in the strakes - whether from moisture content or from wringing - and doesn't rely on adhesion to the wood like a mastic.

Alan D. Hyde
03-19-2002, 04:15 PM
An interesting and instructive post.

Thanks, William.