Treated Plywood and Aluminum Stringers

[cnr2003]

This fall I'm going to be replacing the deck in my 22' aluminum Starcraft. I want it to last a while (the floor in it is 32 years old, I'd be happy with 5 years out of a new floor), but don't want to spend a fortune.

I have already purchased a couple sheets of 1/2" treated plywood and my question is what do I need to do to separate it from the aluminum so they do not react?

I have 3 gallons of marine spar varnish that I picked up for free and I'm wondering if this will be enough of a protection between the two. If this is sufficient should I coat all sides of the wood? The plywood has been in the garage for a few weeks, so it should be dry enough to not have to worry about rot from the inside. Would the floor last if I used multiple coats over the treated plywood?

I don't want to deal with epoxy or fiberglass if I can avoid it.

 

[oops!]

Re: Treated Plywood and Aluminum Stringers

i dont know why everyone missed this one.....

my friend......sorry to tell you.....the pt ply will cause a galvanic responce in your boat.....and eat it .

no kiddin.

i have heard of cases where it hasent (pt coming in contact with alum) but chemestry is what it is.

i know you dont want to fiberglass anything......but a good exterior grade plywood with poly resin will last 20 years or double if done correctly.

the spar varnish you have.......even tho i have not used it.....is not reported on the forum to be a good product for what you are wanting it to do.

read lunds project in the compleated projects forum....that boats gonna last him 40 years.

sorry to be the bearer of bad news.....but rather now than finding out when your boat is done and in the water

cheers
oops

 

[tashasdaddy]

Re: Treated Plywood and Aluminum Stringers

i would return the treated ply, and get exterior AC 1/2 then fit the floor. remove and put 3 coats of oil base paint. the marine spar varnish will be fine, 2nd coat, add a little sand as a non slip additive. it will last for you, if you keep the boat covered, when not in use. most aluminum boat decks are not glassed.

 

[cnr2003]

Re: Treated Plywood and Aluminum Stringers

my friend......sorry to tell you.....the pt ply will cause a galvanic responce in your boat.....and eat it .

I understand that, but I'm wondering to what extent. Using spar varnish, oil based paint, or even epoxy resin adds a waterproof barrier correct? Would any of these be enough of a barrier to separate the chemicals in the PT ply from the aluminum stringers?

 

[BillP]

Re: Treated Plywood and Aluminum Stringers

I understand that, but I'm wondering to what extent. Using spar varnish, oil based paint, or even epoxy resin adds a waterproof barrier correct? Would any of these be enough of a barrier to separate the chemicals in the PT ply from the aluminum stringers?

You're 100% on track. Epoxy (or duck tape, rubber tape, plastic tarp) will certainly provide an adequate barrier for everything but the fasteners. They can be isolated by using nylon washers and bushings. Using proven methods pretty much cancels both rot and electrolysis/corrosion fears.

If you look at what the alum boat industry has been doing for the last 30+ yrs it is often pt ply on alum stringers...with poor results or stellar results depending on how carefully it was built. Those with poor results didn't isolate the pt from the alum. Those who isolated the pt have stellar results. Basically, some build for long life and others don't care or know how to.

My 1996 20' alum boat has 3" wide x 1/8" thick rubber strips under the pt ply floor and it has NO corrosion...12+ yrs without corrosion says it works. It's used in saltwater and sits in a boat lift over the saltwater 99% of the time. When I added u-bolts to the alum for lifting I used the nylon bushings and washers to isolate them too...no problems...12yrs.

How durable is the pt ply on alum? My boat has no paint or coating the underside of the pt ply floor and only glued down carpet on the top. The wood and alum looks weathered on the underside and I can't tell anything from the top other than really worn carpet. No delaminating, structural or corrosion issues.

bp

 

[cnr2003]

Re: Treated Plywood and Aluminum Stringers

BillP you've just made my day. That's exactly the response I was looking for.

I think what I'll do is use the spar varnish anyways since it was free and should help prevent cracking and wear. Then carpet the top and add some kind of spacer between the stringers.

Where did you find the rubber strips?

 

[oops!]

Re: Treated Plywood and Aluminum Stringers

BillP you've just made my day. That's exactly the response I was looking for.

I think what I'll do is use the spar varnish anyways since it was free and should help prevent cracking and wear. Then carpet the top and add some kind of spacer between the stringers.

Where did you find the rubber strips?

happy day for u

bill p is a glass man and he has a super amount of expoeriance....
and td has used just about every product under the sun

 

[The_Kid]

Re: Treated Plywood and Aluminum Stringers

https://www.materialscraps.com/Merchant2/merchant.mv?Screen=SFNT&Store_Code=M

Materialscraps has a lot of different types of rubber that could be used as an insulator between the wood and the stringers.

 

[tashasdaddy]

Re: Treated Plywood and Aluminum Stringers

why not use the insulation roll they put under wall these days. it's 3 1/2 inches wide available at HD, Lowes,

 

[BillP]

Re: Treated Plywood and Aluminum Stringers

Just about any material will work, just make sure it doesn't absorb water. If it does the water will be a conductor and help electrolysis happen. Duck tape or clear shipping tape is fast, cheap and easy for something like this and probably what I'd use over alum deck beams.

bp

 

[109jb]

Re: Treated Plywood and Aluminum Stringers

Look in the electrical dept at the home center. There is a heavy vinyl electrical tape, about 2 inches wide and twice as thick as run of the mill electrical tape. I used rubber strips on my 1974 Starcraft Supersport, but if I were to do it again I'd use this tape and just stick it to the top of the stringers.

 

[Dunaruna]

Re: Treated Plywood and Aluminum Stringers

What about polyurethane caulk, it would provide a barrier AND adhesion.

 

[cnr2003]

Re: Treated Plywood and Aluminum Stringers

Thanks for all the replies. I think I'm just going to use either duct tape or the vinyl electrical tape mentioned.

My only other question is what fasteners should I use? The old floor was secured with aluminum rivets. Could I just use stainless steel screws with the PT plywood or would I still need to use rivets and nylon bushings?

Also, would the polyurethane caulk or some other type of glue be sufficient enough so I wouldn't have to use fasteners?

 

[cnr2003]

Re: Treated Plywood and Aluminum Stringers

After some searching through the forums, I think I'm going to go with Stainless Steel fasteners.

What type of SS screws would work best for attaching the plywood to the aluminum stringers?

 

[fishrdan]

Re: Treated Plywood and Aluminum Stringers

My 01 Crestliner has this type of screw in the decks

Supposed to be called a "wafer head" self tapping screw. Looks like a pretty good design since it will distribute the clamping force over a larger area. I'm trying to locate some for my project....

 

[109jb]

Re: Treated Plywood and Aluminum Stringers

After some searching through the forums, I think I'm going to go with Stainless Steel fasteners.

What type of SS screws would work best for attaching the plywood to the aluminum stringers?

On my Starcraft, I used SS deck screws I bought at my local home center. I pre-drilled with I think a 1/8" or 3/16" drill dit. They worked fine except I had a few heads break off.

 

[marquette]

Re: Treated Plywood and Aluminum Stringers

window flashing tape or door sill tape will stick and last forever if it is protected from UV rays so it would work under a floor. it's designed to self seal when a nail or screw goes through it. you can get it at Lowes, HD or any building supply.