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TOPIC: 4788 rubrail caulking question???

4788 rubrail caulking question??? 03 Sep 2017 17:57 #1

  • Woodsea
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I just removed a thin, partially adhered bead of caulk from the top of the rubrail to hull seam. The seam looks pretty darn tight. I am wondering the neccesity for this bead of caulk. What do you all think?

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2000 4788 w Cummins 370's, underhulls, swim step hull extension
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4788 rubrail caulking question??? 03 Sep 2017 22:17 #2

  • p91473
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My experience is if you do not re-caulk once you are out on a rough day water will find its way in, tight seem or not.
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1997 3788/Cummins 6BTA 5.9 M2s (Sold)
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4788 rubrail caulking question??? 03 Sep 2017 22:30 #3

  • Mr. Darcy
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Clean out all of the old caulk, clean the surfaces as deep in the seam as you can, then PUSH the caulking gun along to fill as much of the gap no matter how narrow it is. Never pull a caulking gun. A pulled caulking gun will not fill the gap, but rather lay a bridge of caulk across the top of the gap. It won't last long. You may already know all of this but it is worth stating for others who may read this thread. Recently did this on Mr. Darcy. There... my two cent comment is done.

Greg
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Newport, Oregon
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1986 3270 with twin 110 HP Hino diesels. Name of boat "Mr. Darcy"
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4788 rubrail caulking question??? 04 Sep 2017 01:31 #4

  • MerlinV
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In a perfect world, all of the fasteners for the rubrail will be totally sealed, eliminating the need for additional caulking.

In the real world, I agree with other posters that an additional bead of caulk along the top edge would probably be the way to go, but only if you prepare the surfaces well.

Chasing a leak last winter, we ended up removing all of the rubrails and starting over, resealing every nut, bolt or screw along the way. We found several holes drilled through the shoebox join that were supposed to have a nut and bolt inserted, but someone 'forgot' at the factory. Upon reassembly, we did in fact lay an additional bead of caulking along the top edge.

Whatever you do, take the time to thoroughly clean and mask for the bead you are applying.
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4788 rubrail caulking question??? 15 Sep 2017 22:36 #5

  • boatworkfl
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Caulking the top is OK, but what about the bottom, water spray will enter through the bottom when running.
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Pat says: DO-IT-RIGHT THE FIRST TIME!

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4788 rubrail caulking question??? 16 Sep 2017 00:38 #6

  • rogerkarenorth
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On a previous boat I calked the rub rail by removing a couple screws, pumping calking in one until it came out the other and then removed the adjacent screw and moved down the line. This forces the calking into the hull deck interface as well.
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1997 4788 with Cummins 5.9 (315 hp)
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4788 rubrail caulking question??? 16 Sep 2017 02:38 #7

  • Cool Beans
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If the rub rail doesn't leak into the interior...I wonder if it is worth caulking. I believe it is an all or nothing caulk job. Water will get in some how and get trapped if you only partially caulk it. I'd venture a guess to say that it might be better to leave it uncaulked so any water might work it's way out.
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. . .It places the lotion in the Basket. . .and that basket happens to be in a 1987 Bayliner 3870 w/ Hino 175's

4788 rubrail caulking question??? 18 Sep 2017 13:08 #8

  • Sunbird
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I am currently replacing the factory rub rail. I plan to remove all the screws holding the joint together and re-install with 5200 to avoid them loosening up. Then I will caulk the seal from underneath the joint and fill all the original rub rail screw holes. Then install the new rail and use sealant on those screws as well. I did not plan on caulking the rub rail.
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Gibraltar, Mi.
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4788 rubrail caulking question??? 18 Sep 2017 14:42 #9

  • boatworkfl
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I would use 4200, 5200 dries hard, 4200 cures with some flex.

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Pat says: DO-IT-RIGHT THE FIRST TIME!

Bayliner 3870 "ALASKA33)
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4788 rubrail caulking question??? 18 Sep 2017 15:17 #10

  • Centerline2
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MerlinV wrote:
Whatever you do, take the time to thoroughly clean and mask for the bead you are applying.


Merlin said the two most important steps of the process that dont get done often enough.....

CLEAN doesnt mean "wiped off"... use lacquer thinner, xylol, or acetone and clean it well and the caulking will adhere to the surfaces for the life of the caulking, or boat, whichever comes first...
make sure the cleaning chemical is completely wiped of and traces evaporated before applying the caulking, because these chemicals can have a reaction with the caulking

MASKING the rail and hull with tape is the only way to get a clean and professional looking job (unless of course you ARE a professional and have used a caukling gun enough to know all the tricks).... the nice thing about masking is (other than it being much cleaner), after masking the area about an 1/8" of an inch either side of the seam, you can push or pull the caulking gun while laying the bead, THEN run your finger down the bead firmly to PUSH the caulking into the seam while at the same time smoothing the caulking.... if you get a spot that is open and needs more caulking, you can easily lay another bead and smooth more of it into the seam without making a mess like is normal when no masking tape is used.....

remove the tape as soon as the seam is filled to prevent a dry tape line that may try to lift with the tape..... then use a wet finger (i like to use a bit of soapy water to wet my finger) and run it very lightly along the still wet caulking to smooth any tape line..... its perfect job every time and much simpler and cleaner to achieve than when trying to caulk without tape..

after masking and before applying the caulking, I use a scotchbrite pad to buff the seam lightly, then wipe it clean again... I feel this insures a longer lasting grip as there is no chance of any oxidation left between the caulking and the gelcoat.
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1989 Bayliner 2556, 5.7 OMC Cobra
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4788 rubrail caulking question??? 18 Sep 2017 16:39 #11

  • W8N4SUN
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Hope this isn't too much of a hijack...while cleaning this area I noticed sealant was peeling away and I'd like to reseal to prevent water getting in while underway. Water hits this area pretty hard at cruising speed (kind of a silly design). Ideally I'd like to put a deflector shield of some sort, in the meantime what is the best sealant to use - 4200? Thanks

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2001 3788 w/ 330 Cummins
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4788 rubrail caulking question??? 20 Sep 2017 00:02 #12

  • Centerline2
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W8N4SUN wrote: Hope this isn't too much of a hijack...while cleaning this area I noticed sealant was peeling away and I'd like to reseal to prevent water getting in while underway. Water hits this area pretty hard at cruising speed (kind of a silly design). Ideally I'd like to put a deflector shield of some sort, in the meantime what is the best sealant to use - 4200? Thanks


if done properly 5200 would be my first choice, 4200 my second choice. because if done properly, it will never have to come apart until there needs to be some reconstruction, in which case grinders will be used... 4200 is a strong but "removable" adhesive caulking, but 5200 is considered a "permanent" adhesive caulking... both set to about the same durometer/consistancy, and remain very elastic and flexible.

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1989 Bayliner 2556, 5.7 OMC Cobra
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