Fiberglass layup over large area

[Travis H]

Hello everyone,

I'm not doing construction on a boat, but a top for a 72 Chevy Blazer that is made of SMC (Sheet Molded Component). I hope this is allowed here. I'm really struggling to find the answers I need from anywhere else.

The tops for these were really under-built from the factory and had many issues. I ended up removing the entire outer flat section on the top because it had so many issues and was the only way I could disassemble it completely to tackle the rest of the repair work inside.

The 'supports' inside were very underwhelming and had far too few for my liking, so I removed them and constructed a metal frame inside and then reinforced the headliner area with foam strips covered in woven fiberglass.

I'm only a week or two away from being ready to lay the new outer skin on the top. I need to order the rest of the fiberglass and resin to complete the job but I'm not entirely sure how it should be laid up. Almost every resource online is working in the opposite direction, starting with a mold and laying fiberglass from the inside. I am working from the inside out. Once the metal is bonded to the inside layer, I'm going to pour foam then shape and cover with fiberglass.


Now for some fiberglass specifics:

The area to be fiberglassed is approximately 5' x 8' (~4.5 yd^2) .

I calculated the weight of the original SMC for this area to be around 1 lb/ft^2, or 9 lbs/yd^2, or about 40 lbs total, so I would like to keep the finished weight close to this.

I'm looking at a minimum of a few layers of biaxial (with no mat backing because I'm using epoxy) and a few layers of thinner woven fabric.

Question 1: is this general layup good or should I make changes? Do I need a layer of woven between the biaxial?

Including epoxy weight here, I could do:
2-3 layers of 24 oz/yd^2 (~14 pounds per layer)
4-5 layers of 12 oz/yd^2 (~7 pounds per layer)

Question 2:
The area to be covered is wider and longer than the fiberglass so I cannot do a continuous layer in any direction. I also require a flat finished surface. How should I handle this?

I had a few ideas. One is to do several layers (say 4) where I overlap 1/4 for each layer. By moving the shifting the location of the overlap, I would end up with a 5 layer thick laminate from 4 layers.

Alternatively, I could simply lay the fabric down and allow them to just touch on the edge. For this I would want at least 3 layers so that at no point is there only 1 layer of biaxial.

What are your thoughts on this?

 

[fallguy]

You intend to bond a skin to pour foam.

The pour foam must be like 4# or better. Lighter foam will delam too easily.

Edges must be filled with heavier than 4# density material, like minimum 20#/sqft.

You are incorrect about csm, here csm would help you get a fast build and make a good bond layer to the foam. Of course, it is super thirsty and you'll need to be around 110% resin to glass by weight for this first layer because the foam will take some.

All fiberglass must overlap 2" minimum as a general rule, but you could put a fairing relief in the foam..it would be sort of hard to do, but maybe an electric planer set at 1/8" would do it.

I'll start with these comments.

The foam...is that going above the crossmembers or to?

 

[Rumars]

Why do you want to make it harder then it needs to be? Attach some wood strips to your metal frame so that they are all level (use a piece of ply to check). Buy some XPS, cut it to fit around your metal frame and level with the wood, then stick it down with some thickened epoxy. Buy a premanufactured fiberglass sheet (or lay your own on a piece of melamine) and glue on top, again with thickened epoxy (stick it to a sheet of ply with double sided tape to keep it flat, protect the ply with plastic from epoxy). Any needed roundings can now be hand laminated with small strips of biax (feather the big sheet). This will significantly cut on fairing.

If you want to laminate with epoxy on pour foam two layers of 6oz is plenty, this is just waterproofing after all, the strenght is in the metal frame. There is no need for overlaps, you just butt the strips and stagger the seams.
In order to get a smooth surface you first fair the foam with a longboard, then fair the glass afterwards. You will end up using a lot of fairing compound, laying glass on foam is never smooth, a lot of sanding will be necessary.
With pour foam I would be using polyester resin, with one layer of 1208 (CSM side facing the foam), overlapped 2inches with rebates sanded into the foam. Then all fairing also with polyester based products.

 

[wet feet]

You are going to be faced with an almighty mess.The combination of gravity and expanding foam will make things really "interesting".Then you will have to create a more or less fair surface to glass over,prior to achieving an acceptable finish on the outer surface.It doesn't end there as poured foam has a tendency to expand if exposed to strong sunlight,which may result in some novel bulges where the thickness of foam is greatest.Does Oregon see much strong sunlight?It might also cause a bit of a problem if the expansion rates of SMC and epoxy/glass are too dis-similar.I have a feeling that better advice might be had from a Corvette forum as they would have more experience with SMC and making alterations.

 

[kapnD]

I’d sure be looking at replacing the sheet metal in kind.
What is the material is under the reinforcing strips you’ve added?
Will it stay bonded as temperatures fluctuate and load is applied?
Why wouldn’t they be continuous across the span?

 

[Travis H]

Thanks for all the replies. There is a lot of difference in opinion and solutions here.

The crossmembers on the headliner area cover the areas between the metal frame work, which is why they are divided into sections and are not continuous. Once the frame goes in the metal will be bonded to the inside layer setting the shape of the inside layer. The crossmembers are made from 6 lb/ft^3 foam that I was planning on using to fill out the rest of the areas (inside crossmembers). I needed to add the crossmembers because the area needs to be mostly flat (I tried several different ways of doing this in one go, but there was always a big hump or sag that couldn't be corrected). Nothing on this project is truly flat, there is curve and bend to just about every panel along with 50 years of sag and damage to contend with. This is includes the outside.

I'm not concerned with the foam being 'messy' to work with. I'm going to fill the sides and corners of the top after bonding the outer layer to the inner layer on the edges and before I fill the headliner area.

The sun shouldn't be a factor in this project. It will be painted white. I generally remove the top in mid spring and replace mid fall, so the top itself rarely sees harsh continuous sun-light. It is also kept in the garage. There shouldn't be a significant difference in the expansion rates of SMC and epoxy/fiberglass. SMC is generally polyester and fiberglass with additional wax and additives so that sheets can be quickly molded in a hot press and released easily.

I like the idea of bonding a pre-made fiberglass sheet but finding something close to the size I need is around $250 plus about $250 to ship. I could make one myself, but I think this will require almost as much work (and more materials I don't have room to store) than if I just bit the bullet and dealt with fairing the foam.

 

[fallguy]

Just use epoxy and a piece of 1208...some 20# core or wood edges by the cutouts, relief the foam up the middle...I like the mat down.

Don't pay any attention to 'can't use csm with epoxy'. The reason people say this is because the epoxy doesn't solve the binder, but it doesn't need to unless fine forming..which you are not.. but mat backed glass will give you something solid-ish over the foam. You can two layer it if you want to walk it..

 

[rnlock]

Seems like the big flat area in the middle could be plywood, with very light glass cloth and epoxy to keep it dry, which has worked for some of my boats for many years.

I admit I'm a bit confused about the details described here. My experience with pour foam is that it's pretty rough stuff, although if it has a heavy layup over it, that might not be very important. I've heard tales about it expanding later on from other sources. It might be a good idea, after any pour, to put black plastic over it and leave in the sun for a while, before shaping it.

If you haven't bought the epoxy yet, it might make sense to pick some that has a higher glass transition temperature. Or, at least, I think that's the term. The temperature when it starts to soften up.

I'm afraid I don't see how making a flat fiberglass sheet could be more work than pouring and shaping a bunch of foam. If you have a flat table covered with something smooth, and a smooth sheet of plastic over that, wouldn't it be easy?