Joining sheet ally to GRP.

[EFA]

On my project I have a need in a couple of places to join 16g sheet ally to 1/8" thick grp in such a way that it forms a contiguous surface. I don't want to rivet it, the finished join needs to be undetectable once painted.

So I was wondering:

Am I having a laugh - the thermal expansion of the alloy realtive to the GRP will cause the join to crack (some of this is above the engine)

Will it crack anyway through moderate vibration?

Can I overlap and secure with sikaflex - what would I use to fill the top of the join?

Should I give up and try to mould the whole lot in GRP?

Thanks

[S47]

Sikaflex or similar is your best bet - eliminates the thermal expansion problem, however you won't get the join perfect so won't be able to spray it again due to the expansion.
If you can mould it completely in GRP then that's your best option IMO - either way good luck

[gtmdriver]

No matter what method you use to join the two materials any filler or paint will eventually crack over the join, so if you really need a continuous surface you will have to make it in one piece. If you choose to stick with the two materials then Sikaflex or one of the structural adhesives such as Araldite 2015 or one of the Crestomer products will work fine.

[dmulally]

I will try and break it down to steps what I would do. Sorry if it is hard to read. In one big paragraph it looked odd.

First, I would scuff up the fibreglass so its very coarse.

Then I would drill holes in the ali. Big ones.

I would use sikaflex 252 to bond the two but be sure to dodge the holes. Be sure to have the ali scuffed up as well so the glue sticks better.

Then I would use a flapper disc on a grinder and mess the ali up something bad. Perhaps with 40 grit.

I would then go over the join with lots of resin and wovern matting. Maybe one layer. Any more and it will be too high. Being sure to press into the big holes.

Once that is all done, I would smooth out the massive high points and go over the lot with fibreglass re-enforced filler as the wovern matting is pitted.

I did that with the flares on my 23 arches and it has held up really well so far although it has only been a year. I hope this helps.

[Slow M]

May 2005 Racecar Engineering has a good article on this. Do you have access to it, or would you like a scan?

[Incorrigible]

I don't know what level of finish you're after but you could stop the paint cracking by adding a plasticiser to make the finish more flexible

the post a couple up makes sense but would of course add weight, I don't know if this is important in this application

Or just get someone properly talented to knock one up out of sheet ally