Layering Fiberglass. Knowing when to stop

[mfleming]

I have searched VAF and the web and haven't come across a explanation on laying fiberglass in layers.

I'm ready to build the fiberglass skirt on my -7 slider. it's my understanding that I can lay down cloth and butt each section to the other and the next layer stager the joints for strength.

As long as the epoxy resin hasn't started to kick, I can keep going.

My question is: Can I continue to add fabric when the previous layer has started to gel? If not, do I just stop and add peel-ply and pick up where I left off after its cured?

[scsmith]

You can do it either way:

stop, add peel ply, allow to cure, then scuff sand and continue.

or, just keep laying on more plies. Unless you are going to vacuum bag it, you don't really care that it has started to gel down in lower layers as you add more layers.

Epoxy has a long working time though. We do very large layups (like a whole wing skin or a fuselage half) with rarely a problem. It starts to gel quicker in the mixing cup than on the work piece, so for large items, we just pour the resin out onto the work piece and spread it around rather than brush or pour small amounts out of the cup as needed. The sooner you get it spread out thin, the longer it will take to start to gel. It also helps to do it when it is cooler.

[RVDan]

When doing wet layup for non-structural parts the number of layers will depend on the cloth weight. The goal is to keep the resin to cloth ratio as to a minimum without having a dry layup. What happens if the buildup gets too thick is that it will get squishy, indicating too much resin. From that point on the layers will become easy to move around and will contain lots of excess resin. Best to apply peel ply and let it cure before that. After the resin cures, remove the peel ply sand off irregularities to allow the next layer to lay evenly and continue. No need to sand off the peel ply print as it provides a sound bonding surface.

BTW, in my experience, it is typically 3-4 layers depending on cloth weight.
Hope that helps.

[PilotjohnS]

How many layers do you plan to use?
I used 5 layers of 9 oz for the windscreen skirt, but only 4 layers for the aft skirt. I hope this is enough for the aft skirt; I wanted to do 5 layers on aft, but the front came out really stiff with 5, and I ran out of glass cloth when doing the aft.

Do you all think 4 layers is enough for the aft skirt?

To answer your question, I laid up two layers each session, no peel ply. The windscreen's first two layers I added black dye, the last three layers no dye. I popped it off the plane and trimmed it, then I added a coat of epoxy and peel ply.

For the aft, I laid up the four layers over two sessions, and the last session I laid down peel ply. Also, as I was about to lay down a new layer, I lightly brushed epoxy on the previous layer before laying down the next layer.

[mfleming]

Originally Posted by PilotjohnS

How many layers do you plan to use?
I used 5 layers of 9 oz for the windscreen skirt, but only 4 layers for the aft skirt. I hope this is enough for the aft skirt; I wanted to do 5 layers on aft, but the front came out really stiff with 5, and I ran out of glass cloth when doing the aft.

Do you all think 4 layers is enough for the aft skirt?

To answer your question, I laid up two layers each session, no peel ply. The windscreen's first two layers I added black dye, the last three layers no dye. I popped it off the plane and trimmed it, then I added a coat of epoxy and peel ply.

For the aft, I laid up the four layers over two sessions, and the last session I laid down peel ply. Also, as I was about to lay down a new layer, I lightly brushed epoxy on the previous layer before laying down the next layer.

Five layers is what I had in mind. The way I was taught to think of it at the EAA Composites for RV class was relate it to aluminum. So 4 layers would be close to 0.040" and five layers would be 0.050". The metal skirts are what...0.032" I think. So either 4 or 5 layers sounds adequate.

So I think the way I'm going to lay up my skirt is:
Two layers sandwiched in plastic with the excess epoxy squeezed out. Butt joint each strip, add peel ply, let cure. Next day add two more layers the same way and stager the joints. Evaluate the thickness and if needed add one or two more layers

[mfleming]

Originally Posted by RVDan

When doing wet layup for non-structural parts the number of layers will depend on the cloth weight. The goal is to keep the resin to cloth ratio as to a minimum without having a dry layup. What happens if the buildup gets too thick is that it will get squishy, indicating too much resin. From that point on the layers will become easy to move around and will contain lots of excess resin. Best to apply peel ply and let it cure before that. After the resin cures, remove the peel ply sand off irregularities to allow the next layer to lay evenly and continue. No need to sand off the peel ply print as it provides a sound bonding surface.

BTW, in my experience, it is typically 3-4 layers depending on cloth weight.
Hope that helps.

Thanks for the info...really helped in making my decision on moving forward.

Can one add SuperFill or epoxy/micro-ballons between cured layers? I'm assuming I can.

[nohoflyer]

FYI, the current technique being taught at the EAA fiberglass class is to use a base layer of micro and flox followed by two layers of cloth followed by another layer of micro/flox.

They much preferred that to the multiple layers of cloth at the windscreen/metal mating.

[mfleming]

Originally Posted by nohoflyer

FYI, the current technique being taught at the EAA fiberglass class is to use a base layer of micro and flox followed by two layers of cloth followed by another layer of micro/flox.

They much preferred that to the multiple layers of cloth at the windscreen/metal mating.

Since I'm going to finish the one piece skirt off the canopy, then sikaflex it on, there will not be a base as described.

I guess what I'm asking is can I touch up irregularities between layers or should I just get all the layers laid down and then address the highs and lows then?

[wirejock]

Originally Posted by mfleming

Since I'm going to finish the one pice skirt off the canopy, then sikaflex it on, there will not be a base as described.

I guess what I'm asking is can I touch up irregularities between layers or should I just get all the layers laid down and then address the highs and lows then?

I wrote a series of articles for Kitplanes on the subject.
I prefer one layer at a time to keep excess epoxy under control. Two layers tops then peel ply to squeeqee the layup. Next day rip it off and continue. Mine is five layers. Typically .010" per layer. Edges overlap and stagger in different places. A few spots were irregular so I applied flox then the next layer and peel ply. Squeegee and the flox flows underneath into the low spots. Still retains it's structural strength. Feel free to call if I can be of any help. I also have an epoxy calculator on my blog.

[RV7Guy]

You mentioned the canopy skirt. Are you taking about the side skirt or the rear one? If the rear one, you really want to bond it to the canopy during the lay up.

I did mine as follows. I am a proponent of doing one layer at a time. 1) taped down a layer of thin poster material over the turtle deck. Then taped over that as a release agent. 2) The canopy was thoroughly scuffed with heavy grit paper. Actually turned the canopy white. Clean 3) First layer was carbon fiber as it is easier to comply with bends and curve. 4) Second layer was 6 ounce S glass. FYI I used peel ply on each layer. 5) After the second layer was applied and cured it was sanded and the process of filling began. Epoxy and micro filler was used. After cure, sanded and more fill as needed. Cure and sand. 6) one layer of glass was added with peel ply. Cured, sanded and filled as necessary. 7) 2 layers of carbon were added individually with peel ply.

This was all the layers needed. Now I filled with Evercoat 416 glazing compound as very little was needed. Everything was block sanded to final form. When finally done I measured and marked to desired width. (I actually used a micrometer to insure it was the same) Once the width was established, I rounded the trailing edge of the work.

FYI, I used the stock metal side skirts but they were Sika's in place. No holes for fasteners in my canopy. I did bond a piece of 2 ounce carbon fiber to the side skirts. The only purpose was to serve as the shadowing for our painted on names.

Turned out pretty good