How many layers of fiberglass for strength?

[Linlor]

In all my fiberglassing work, I use 1.5 oz. matt and use 4 layers minimum for flat surfaces for strength.  If you want, you can also add some triangle tubes on the top of the panel to add additional strength without the added layers and weight.  The strength comes from the angles of the fiberglass laid over the triangle tubes and reduces some of the weight you would end up with by adding additional full layers of matt.

Once you start getting up to 4 layers of glass though, the flexibility starts to go away and the panels become more rigid so if you need to have a shape to the part, lay up two layers of matt as per the response above with non waxed resin, and set the panel to the shape required and make sure it stays in place.  Either in the location where you want it to be fit or in a jig of sorts to keep the shape.  Add an additional 2 layers min of the 1.5 oz. matt and waxed resin to the panel in its shape and let it set for a day or so.  Trim the panel and install if it wasn't already in place.

Hope that makes sense.

Cheers
Don

[01Lambiero]

Although I am not an expert either   If you watch the "I've got a hole in my boat" series on Youtube, you can learn from an expert. 
Use laminating resin for layup work. (resin without wax additive). Resin coat, cloth fiberglass, resin, chop strand, resin, gelcoat with wax. 
IF you are making panels on glass (flat surface), reverse the process.  wax the glass 10+ coats of wax (15 min. between coats) apply gelcoat without wax additive (gelcoat and resin comes with or without the wax additive.  Resin and gelcoat will not cure when exposed to air.  The wax seals the surface so that the resin/gelcoat will cure.  You can layer over resin that has wax but the entire surface must be sanded) resin, cloth, resin, cloth, resin, chop strand, resin with wax (resin without wax additive will cure if sprayed with PVA [mold release agent] to seal it from the air.  If you want a stronger layup, add extra layers of cloth.  NO AIR POCKETS OR AIR BUBBLES.  Apply with throw away brush and roller out extra resin.
Clear as mud??  Please watch the videos #1,#2, Hole in my boat.

[italianknightrider]

if 1.5 oz 4 would be good ,if its thin it could be 3/4 oz requiring more ,be sure to rough it up , with some 36 grit sand paper or grinding disc so you get a good bond of the new GRP TO OLD.

[lance345]

Hey all,

I'm hoping someone that has more experience than myself can give me some advice. I am using chopped strand mat fiberglass (I don't know what weight, the place in town only has 1 weight but it is thin, maybe 1.5oz?). I made a huge flat sheet with just 1 layer to make some filler panels while it's nice and flexible, but now I need to reinforce them.

I'm using a 2 part epoxy, and they don't need to support weight, just be rigid. I'm leaning toward 4 layers, out to lunch?