Sealing deck screws

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Lipidman

New member
This is pretty elementary but important to me. What should I use to seal deck screws (eg navigation lights, rod holders)? A West Marine salesperson told me to use 5200 but didn't seem very sure. I want to be able to remove them. Thanks, Roger
 
5200 is quite permanent. I would not use it, rather I'd use 4200 or another marine caulk/sealant. I always try to make sure the hole is clean and dry and, if it is to be a new hole, bevel the edges very slightly to reduce the likelihood of gelcoat cracks as the hole is stressed from inserting the screw.
 
Lipidman":3mbaj6do said:
This is pretty elementary but important to me. What should I use to seal deck screws (eg navigation lights, rod holders)? A West Marine salesperson told me to use 5200 but didn't seem very sure. I want to be able to remove them. Thanks, Roger
Click to expand...

It can be confusing, can't it?

I call the final step (of using some sort of compound on fasteners and fittings) "bedding." Bedding helps to seal the hardware (and must be renewed periodically), but it may not do the whole job. More:

What I do is first figure out what is needed at a particular location. For example, is the fitting resting on solid fiberglass? If so, bedding alone may be fine. Or is it (most common) penetrating a cored surface? If it's penetrating a cored surface, then I like to take extra measures to prevent (or, sadly, sometimes reverse) damage to the core. A wood core can get wet and rot; a foam core can get wet and de-bond (skins separate and strength is then lost).

The basic idea is then to "overdrill" the hole (make it larger than needed), fill in that hole with thickened epoxy, and then re-drill the correct sized hole through that "tube" of hardened epoxy. At that point any bedding compound used will simply help to prevent water going down the hole (along the fastener) to whatever is below it; but the epoxy annulus will protect the core.

As far as what to choose for a bedding compound: There are many choices (none perfect), and various reasons for choosing one or another. Factors I consider:

1) When the time comes to re-new this, how hard will it be to get the old stuff off? (For example, silicone pretty much never goes away; 5200 [a strong polyurethane] can be hard to remove.)

2) Will this be exposed to UV? Will that have a negative effect? (For example, white polyurethane can yellow with UV exposure.)

3) What about solvents or fuels? (For example, butyl can dissolve in the presence of fuel.)

4) Do I want a pure bedding compound? Or am I looking for an adhesive too? (For example, butyl is more of a pure bedding compound; polyurethane can be very adhesive.)

5) What about any mess during application? Do I need something really neat? Can I tape off effectively?

6) Do I have temperature constraints?

A few ramblings:

Basically, there are a few "families" of caulks/bedding compounds, and various brands within them. But people tend to use the brand names at times instead of the base name, so it can get confusing.

1) Silicone
Let's just get this out of the way first :wink I detest silicone and won't use it. I don't think it's a great bedding compound/caulk; but worse, it leaves an insidious residue (after you laboriously try to scrape it off without damaging the underlying surface) that can make it nigh on to impossible to ever paint or even re-caulk a surface :amgry

2) Polyurethane (in various degrees of adhesiveness)
This is things like 3M 4200 and 5200, Sika 291, and others. I don't have anything against a strong polyurethane (e.g. 5200), but I don't use it indiscriminately, because it can be very hard to remove (it's very adhesive) and so why struggle with that if those adhesive qualities are not necessary for the job. There are less-strong polyurethanes, such as 4200, and Sika 291 (you can find charts with the actual specs on adhesiveness, etc. if you are feeling data nerdish). Polyurethane can be a bit messy to apply/work, and it can yellow in UV.

3) Polysulfides
This is things like Boat Life's Life Calk, and the (no longer available) 3M 101. It's not very adhesive, is a good choice on wood (but fine on other things too), and is easy to remove/clean up when the time comes to re-bed.

4) Butyl
Butyl comes in either a caulk tube or as "tape" on a roll. It lasts a looong time without losing its elongation/elasticity and is easy to clean up (and easy to work with in the tape form). This is my "go to" bedding compound. It is vulnerable to solvents, so I won't use it on my fuel fills (will probably use polysulfide or a "weaker" polyurethane, but first I'll have to refresh my memory on those, and maybe check into 3M 4000 too).

5) 4000
3M has a relatively new product called 4000, that from what I understand performs somewhat similarly to a polyurethane, but doesn't yellow in UV. I have not used it or researched it yet, and I'm not even sure what "family" of ingredients it belongs in.

Sunbeam
 
In the last year I have been using 3M 4000 it is about the dry time and adhesion/shear strength as 4200, but is UV resistant (does not yellow or break down). I would recommend it as a sealant for bedding screws above the waterline. You have lots of very good suggestions from above. A counter sink will prevent the cracking--running the drill bit backward to start with helps also

As for the type of material 4000 is: it is listed as a Hybrid--and the actual content is:
Calcium Carbonate
Polyether - N.J.T.S. Reg. No. 04499600-6434 (trade secret)
Diisodecyl Phthalate
Titanium Dioxide N-(3-(trimethoxysilyl)propyl)
ethylene diamine Dioctyltinbis(acetylacetonate)
(plus minimal amounts of other solvents)
 
Since it's raining here, I'm doing some Internet researching. I wanted to get some caulk (other than my usual butyl) for the fuel fills, and I looked up 3M UV4000 to see if it was fuel resistant. I didn't find anything specific in 3M's data (unless I missed it), but they don't mention solvent resistance as a plus, either.

http://multimedia.3m.com/mws/mediaw...6EVuQEcuZgVs6EVs6E666666--&fn=60440094930.pdf

I checked on Jamestown Distributors' site, as they often have good information on sealants, and they say "Does not resist chemicals- for chemical resistance try 3M Marine Sealant 101" [101 being a polysulfide]. So I guess at the moment I'm not really sure whether 3M4000 is gasoline resistant ("chemicals" seems a bit broad).

I still have time as I'm not ready to re-install/bed the fuel fills yet, but this thread reminded me that I want to choose/buy something for that job.
 
Wow. Thanks for taking the time to provide such complete answers. The information will certainly help with current projects as well as those in the future. Thank you all very much. This a truly great website.
Roger
 
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