Fixing Aluminium to plywood

[Muggnz]

I've been doing a little research to see how best to fix Aluminum sheeting to my plywood frame.


So far the best I've seen is
http://www.tnttt.com/viewtopic.php?t=32439&highlight=aluminium+screws&sid=a05963092f965f70353b68510d35f197
and numerous pages on what type & thickness of Aluminum to use.

Hence my need to ask . . .
What ( in your opinion ) is the best way of fixing Aluminum to plywood? And why do you recommend that?

Also it'd be helpful to know how far apart these fixings can be.

tia

[toypusher]

If you are putting AL over plywood, you do not have to use anything but the trim to hold it in place. I used outdoor carpet adhesive and put in on just like I would if putting carpet down, with a notched trowel as per instructions on the can. Then I put the AL up and the trim on.

[madjack]

david, I don't use anything at all, relying on the trim, fenders, lights, etc. to hold the AL in place...I do use a couple of pneumatic staples in strategic places to hold the AL in position until the trim is applied(covering the staples)...I do it this way to help prevent "oil canning" which is where the AL "bubbles" due to expansion from heat...IF you use an adhesive, it can cause the bubbles to become permanent...if no adhesive is used, the expansion can spread over the whole sheet lessening the severity of the bubble and allowing it to disappear when all cools down...if you just gotta have an adhesive, we have found that an outdoor carpet adhesive used around decks and pools seems to work the best with the fewest complaints............
madjack

[Muggnz]

MJ, thanks,

That still eaves the question of how best to screw and/or rivet the ali to the ply. How far apart they should be ? How big & what shape is best or worst? I know I'm being somewhat pedantic, but don't want to make many more mistakes, as they'll start showing up.

I found this chart ( sorry about the poor quality )


It seems like I may have to use stainless screens with something to seal them from the ali. Although I'd like to think that, I could find some hard ali screws. They'd have to be a lot harder than the soft brass screws sold here.

PS I have the perfect product to seal the trim from water ingress. Shell Tixophalte. It hates sunlight & setting. yup, it never hardens.

[madjack]

david, use stainless screws...I don't know how they size screws way down your way BUT, I like a #8x1" screw since #6s tend to break or strip and #10s are just way overkill...I have had no problems with bi-metal reaction between the SS&Al...I wouldn't go more than 4"s apart and on critical areas, I go 2"s apart...this is probably overkill as far as fastener strength goes but we are also talking about sealing here as well...I predrill all my screw holes and put a dab of sealant in each one before installing screw...this actually insulates between the SS&A from each other as well as assuring a good seal which won't wick water into the screw hole...the only place I use rivets is on the hatch edge...since all I have out there is 1/8th ply, there is no meat for screws...for this I use a flat head AL pop rivet installed from the bottom up...this allows a flat for the hatch seal to go and since I use an insert molding, it covers the tit on the rivet...if you are using a flat molding, you might have to use an aircraft rivet and rivet gun with a bucking bar.....
madjack

p.s. be wary of using a sealant that never sets...I used butyl rubber caulk on my first tear and thought...Yeah, this is the stuff...a year latter, it was still oozing out of the joint, making a mess...I find a 1 part urethane sealer such as auto windshield sealant or SikaFlex 221 or 3M 5200 to be excellent sealants and even a siliconized acrylic adhesive to work pretty well........MJ

p.s.s. I don't know if the have locations in Oz but FASTENAL (they're international) is a good source for SS screws..........MJ

[Muggnz]

mj,

many many thanks, that answers my questions better than I'd hoped.

SS screws are readily available, at varying sizes, costs & quality.

I'm picking that Tixophalte should do the job. As I intend to place cappings around all the edges of the ali sheets. ie front, top, rear & bottom && the doors. I think there would be less chance of a reaction between the 2 metals if the hole was slightly larger than the screw. Allowing for protection from the sealant.

And after re-reading the specs, it does cure
"Shell Tixophalte has a number of product properties that contribute to its versatility and popularity as a sealant, adhesive and filler including:
o Direct adhesion, applied and sticks directly to the substrate.
o Permanently flexible and won’t crack.
o Applicable under water, ideal for damp and wet conditions during application and over its life-time.
o Latest formulation has improved water repellence and quicker and improved curing under water.
o Non-Harmful, doesn’t react to or damage the surface it’s bonded to.
o Direct application without heating, at ambient temperatures ranging from +5°C to +40°C.
o Protection against rust and humidity.
o Meets the ISO 11600 standard which defines the physical properties of Building Sealants including anticipated movement.
o A Smooth finish after application.
o Stable at temperature range –35°C to 100°C (depending on load conditions).
o Colour - black.
o High flow-rate from aluminium cartridges.
o Long shelf life in aluminium cartridges (at least three years).
"

It's also cheap @ $12.90 per tube

However I do think that a test piece is probably in order. As to date my main use of it is in horizontal areas. It does get all day sun heating it up, under the paint, so I expect it become more softer & more flexible in summer.

[madjack]

I just googled Tixophalte...it looks to be some serious stuff ...............................

[Muggnz]

It's taken a while, but my ali should finally be delivered next week

In the meantime I've had s a small holiday from both the TD & life. AND bought a decent vehicle for towing it & other stuff.

To save time & workmanship challenges, I'm having the trim pieces specifically & professionally shaped to the same size as my TD. I supplied the necessary templates.

I even had time to find a great local place for getting screw'd. I bought 500 stainless ( 304 grade ) screws for around NZ$85. As opposed to the local versions of walmart whom most generously charge around $50-65 per 100 . And the heads of mine, are oversized in their width, but the same height Perfect for allowing more grip.

I also found http://www.anzor.co.nz/?t=112 . The red seals to the right of "Self Tapping (Pk) screw or bolt sealed and insulated. Normal or countersunk application." cost 55c each. So I passed on them.

At the end of all this I've started wondering how big I should make the screw holes to hold the ali in place. Should they be the same size as the screw shaft? Or wider to allow for the expansion & contraction of the ali in sunlight, without ripping the endgrain plywood apart?

[alffink]

David

Unless I missed something and I often do...
I would mount the screws through the "Angle" trim so that it would go into the side wall, I would not put any kind of hardware, into the end grain, may hold for a short while, but in the long run....it will let go, eventually.

Not sure about the adhesive you were discribing, the trim, the doors, the windows is all you really need to tie the aluminum to the trailer, many of us, used a light contact cement to temporairily hold the aluminum in place, but alow the temperature differential to break the aluminum free and alow it to float free of the wood surface.

again this is just my opinion, worked for me, but others may differ.