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Old 07-16-2009
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Best suggestion for a gasketing material to separate steel and aluminum?

Yesterday, after several dozen hours of reciprocating saw work and about ten blades, I finally disconnected my aluminum pilothouse roof from the steel flange to which it was glued and bolted.

I did this to allow the removal of the engine, batteries and old tankage from my engine bay.

The job was considerably complicated not by the 40 or so bolts used (although a few of them had corroded in place), but by the use of a thick bead of 5200 sealant used to make the job what I guess was supposed to be "bulletproof".

Ha.

Anyway, here's my plan for when I restore the aluminum roof.

1) Clean every trace of 5200 from the roof and from the inward facing, 5 inch wide steel flange.

2) Grind the flange top fair and rust free. Tape the flange edges in preparation for painting.

3) Prime the flange with two coats of Metal Prep.

4) Paint the flange with two coats of two-part epoxy paint, the same colour as the rest of the deck.

5) Place a rubber or rubber-like gasketing material in a 4 inch wide strip along the entire top of the flange (about 25 feet all the way around). Drill holes at the appropriate spots through the rubber. The purpose of this strip (1/8" thick) is to provide a compressible, flexible barrier to galvanic corrosion or interaction between the dissimilar metals, both of which will be carrying electrical wires for lighting and/or instruments. Most of my sawing was through "welds" formed by aluminum being bolted to steel. Never again!

6) Lay a bead of NON-tenacious sealant (like 4200 or 231 or even some kind of exterior silicone) on either side of the rubber gasket strip. This is to prevent water ingress from the outside, and condensation from the inside.

7) Bolt together with Tuf-gel or some other appropriate substance to allow corrosion-free fastening with the prospect of an easy removal in the future, if necessary.

My question is with the gasketing material.

What should I choose?

I can cut strips of neoprene or even the less gooey sort of butyl tape. I could use some sort of plastic batten or sheeting, but I suspect that would grow brittle over time (this roof gets warm in the sunshine).

Any suggestions, sources and prices? Thanks in advance to the bedding and sealing gurus!
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Old 07-16-2009
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i dont know what would be perfect, but pvc sheet that is used to make ceramic tile showers is pretty cheap. its gray and comes in 3 foot wide by how ever long you want. it might not be perfect but it would be pretty cheap and work

edit home depot and lowes carry it
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Old 07-16-2009
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butyl tape

Don't forget the omnipresent gray butyl tape as a sealant.
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Old 07-16-2009
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Yes, I was debating using butyl instead of a bead of caulk instead, but first I want to focus on the "get the alu and the steel completely away from each other while isolating the 40 bolts and nuts".
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Old 07-16-2009
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Quote:
Originally Posted by scottyt View Post
i dont know what would be perfect, but pvc sheet that is used to make ceramic tile showers is pretty cheap. its gray and comes in 3 foot wide by how ever long you want. it might not be perfect but it would be pretty cheap and work

edit home depot and lowes carry it
PVC might melt in the tropics and it certainly rots in the sunlight. I don't know if the thickness would suffice. Basically, I'm looking for a thick, wide, not necessarily adhensive tape I can drill through in spots, and cut to slightly overlap at corners so the steel and aluminum never touch.
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Old 07-16-2009
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I would go with the neoprene. Good price, comes in sheets, great insulator and it is effectively out of the sun as it is compressed between the deck and pilothouse. 1/4" would probably be a good thickness as it will end up being about 1/16" after compression.
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Old 07-16-2009
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I believe EPDM rubber would be a better choice as it is very UV resistant and won't deteriorate as quickly at the outside edge. It can not tolerate any oil though. For the bolts you want use the special plastic washers with a collar that goes down in the bolt hole to isolate the stainless bolt completely from the aluminum. McMaster-Carr sells these.

In my job we install bulkhead fiting in plastic tanks using rubber gaskets, then fill the tanks with 12 feet of waste water. We have found that a gasket alone without sealants of any kind will seal well when the thickness of the gasket is at least twice that of any bumps on the surface. So a 1/8" gasket requires a surface with not more than 1/16" of variation from flat. But a 1/2" gasket will take up 1/4"! We often need that 1/2" gasket.

Hoppe this helps,

Gary H. Lucas
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Old 07-17-2009
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Try a product called Bichithane, used in construction. It is used to seal roofing edges and other areas. It comes in varying widths, has adhesive on both sides, and will definitely separate the two metals. I'm not 100% sure on the spelling, but it sounds like "bitchithane". Your local building supply will have some. Give it a look.

Bill
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Old 07-17-2009
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Thanks for the suggestions. The nylon washers with the added bushings sound promising, as does the "bitchithane" and the EPDM rubber, both of which are new to me.

Thanks, guys!
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