caulk/glue for windshield gap

[jimgreen]

I have the common problem of a sizeable gap between the front of the plexi and the forward skin on my tip up.
What are my choices for a caulking or glue to fill the gap? Also, how do you prevent the caulking from squeezing under the plexi and making a mess on the inside. It seems pretty difficult to tape the gap on the inside.

[robertahegy]

If you haven't already drilled holes into the canopy, you should strive to get the edge of the canopy to lie flat on the skin. It takes a lot of effort to get the canopy shape to fit properly. Avoiding the use of filler in this area will allow you to minimize the amount of fiberglass needed to fair in the canopy. I actually beveled mine to allow the edge to be in full contact with the skin. This area gets a lot of flexing and thick fillers and glassing are prone to crack as time goes on. Time spent here is well worth the effort.

Roberta

[Brantel]

This canopy is a mystery...You can have everything fitting perfect and then something will move. You might find after getting the fit as close as you can, when you put the screws/all the rivets in, you might get a gap or two as the canopy moves around.....

What I did and suggest is:

Go to Advance or AutoZone or whatever you have in your area and get some black urethane windshield adhesive/sealant. Comes in a tube like caulk. Lay down a fat bead of that stuff before your final install of the glass and set the glass right down into that bead. This will fill any gaps and give your front windshield nice support and will also glue the front to the skin. It also makes a nice bead around the inside that looks better than the raw plexi edge sitting on the aluminum.















Once this stuff cures, you can then apply your glass like normal. Mix some black epoxy die in with the first layers of glass and that will black the glass out on the inside so you don't see it.

As always, your results may vary and do at your own risk...

[prkaye]

I used a pretty thick bead of West Systems 406 (colloidal silica - it's structural filler) directly along the windscreen-skin transition. I used my finger to round this into a smooth transition (i.e. apply it like caulking). Then I did my layers of glass strips right over this (scuffing up the plexi and metal first where the glass is laid down). Finally i finished with a few applications of West Systems 410 Microlight fairing filler, sanding between applications. I've found that between the 406 and the 410, you can pretty much use either or both of these for almost everything on the plane that requires filling or bonding. The 410 is just awesome stuff and so nice to work with.

Before putting anythign down, i used small clips made from aluminum (rivted into the skin) in a few places to hold the windscreen edge down, and I smeared the bead of 406 right over these and then glassed them in. They're completely buried now and you'd never know they were there. To hide all of this I painted the lower edge of the INSIDE of the windscreen black (extending about an inch up from the edge).

[trib]

I would avoid using an automotive filler in this case. It does look nice, but it will not bond to the fiberglass and therefore will weaken the assembly. As Roberta said, take your time and get a good fit here. This is an iterative process but is not difficult. After getting the correct fit, you should have no more than 1/16" inch gap along the plexi to metal. Mix up a batch of flox (epoxy and cotton fiber or west structual adhesive) and squeegee this into the groove to form a nice transition between the bottom of the plexi and the skin. This is done the first step in laying up the strips from the plexi to skin. You apply the wet cloth directly over the flox and the assembly cures as a single unit. Make sure you use the structual adhesive and not just the filler. This will be very thick- think peanut butter consistency- and won't run in this thin of a gap. Once you paint the assembly you'll never know how it was laid up. Don't even think about the cosmetics here as they don't come into play.

[Brantel]

The bed of adhesive I mention above is not intended to bond to the fiberglass but instead is a bed for the windshield to give it a 100% attachment/support to the skin. No matter how hard you try, little gaps seem to pop up between the windshield and the plexi after all the rivets and screws are in. It will change, I guarantee it.

Do not allow the urethane to extend above the edge of the plexi or to far out from under the edge on the skin.

The glass will have plenty of bonding surface area on the edge/top of the plexi and on the skin.

I still recommend a good Flox mix to fill the transition between the edge of the plexi and the skin. One of the pics above shows this layer (5th one down).

Here is another. Very little of this layer touches the adhesive.



My glass would shatter in a million parts before the front of mine would come loose from the skin.

Cosmetics do apply to the inside where the plexi meets the skin. This is a very visible area that is hard to do anything about after the plexi is attached for good.

Originally Posted by trib

I would avoid using an automotive filler in this case. It does look nice, but it will not bond to the fiberglass and therefore will weaken the assembly. Don't even think about the cosmetics here as they don't come into play.

[jimgreen]

Many thanks for the replies. There are obviously a number of different ways of dealing with this. I'll probably use a bead of the black goop, though I am nervous of smudging it and having a mess on the inside.
I didn't mention in my first post, but I have drilled the holes - that is when this yawning gap developed! I don't think my duct tape held it tight enough.
As they say, "my next airplane will be better".

[Brantel]

It was not that bad. You will need a helper in order to set it straight down into the bed. Once it sits into the goo, don't touch the inside. Just leave it alone and it will look nice. The more you mess with it, the more likely you are to make a mess. The outside can be taped off to prevent this.

Your not the first or the last to have it (the canopy and it's fit) change on you at the last minute. Someone could build 10 of these things and I bet every one of them would do something different.

Originally Posted by jimgreen

Many thanks for the replies. There are obviously a number of different ways of dealing with this. I'll probably use a bead of the black goop, though I am nervous of smudging it and having a mess on the inside.
I didn't mention in my first post, but I have drilled the holes - that is when this yawning gap developed! I don't think my duct tape held it tight enough.
As they say, "my next airplane will be better".

[9GT]

I can not recall reading before if people have tried using the automotive windshield adhesive mastic to set their canopy glass. It was my intention to look further into this idea when I got to that point in the build. If you have ever tried to remove a windshield that is set in this stuff,,,,there is no way its coming out without cutting it out. Seems like it is perfectly suited to airplane building providing a secure leak free bond that can flex with expansion/contraction the canopy experiences, yet can easily be removed should the need arise to ever replace the glass.

[Brantel]

If your going to glue your canopy on...Do a search for Sikaflex.. Hundreds of post on that method.

I would not use the urethane windshield stuff for attaching exclusively as that has not been proven on airplanes but the Sika has.

I got the idea to bed my plexi in the windshield stuff from several other RV builders in my area that did the same. Mine is also fastened with the screws per the plans.

Originally Posted by 9GT

I can not recall reading before if people have tried using the automotive windshield adhesive mastic to set their canopy glass. It was my intention to look further into this idea when I got to that point in the build. If you have ever tried to remove a windshield that is set in this stuff,,,,there is no way its coming out without cutting it out. Seems like it is perfectly suited to airplane building providing a secure leak free bond that can flex with expansion/contraction the canopy experiences, yet can easily be removed should the need arise to ever replace the glass.