ill try to keep this short, the basics the way I think of it is this.
chop strand or mat looks like spaghetti, the glass is like long threads and can be in continuos or chopped variety. since its non directional its a good print blocker and if weight is not an issue you can lay this up really thick and it makes the cheapest strongest but not lightest glass. Like a 19 foot searay boat is prob between 3/4 to over an inch thick, its heavy and strong as steel. this I use for print blocking and filling uneven areas or tight places to sort of smooth them out so you can lay a woven over it easier. since the strands are not straight its strength non-directional. its strength will come from building it thick. but for us its better as a first layer to smooth out irregular surfaces and block print. tear this into small pieces to build up tight areas or go over corners. 3/4 oz is pretty thin, I like 1 1/2oz but comes in other weights/thickness. the glass is labeled by how much it weighs per square yard.
next you have cloth. comes in all sorts of weaves. usually a 0-90. but can be made in many configurations. some weaves are more pliable due to how they are configured. meaning they will conform to more curvy surfaces easier than a stiff straighter weave. for thinner parts or something you might want to flex or if you have a surface with curves these will lay easier with thinner oz than a heavy reinforcement. surfboards get a thin cloth like 4oz and since all the strands run 0-90 that means half go north and south and the other goes east west. this gives you tension and load spreads in those directions. so now you can see how a nondirectional like chop vs cloth that has glass strands that go from one end of a part to the other absorb impacts or hold weight and disperse that strain by the way its designed. I use 4, 6 and 10oz. glass is strongest when it is straight as can be so by weaving over and under you bunch up the material and lessen its strength in this way. the benefit is its ability to conform to curvy shapes. also you can lessen this bunching by weaving where is skips over top 2 to 6 strands before going under. these can also be woven in any number of directions like +-45.
next you have a combination of these which is like the 1208 or 1708 where you bond a chop with a woven. this gives you the benefit of both types and is usually used to build up laminate. for example you could use several layers of 6 oz cloth or one layer of 1208. so hopefully you can start to see based on what your needs are that you have materials with specific properties that allow you to choose a layup schedule to fit those needs.
then you have the carbon and kevlar. which are stronger strands per oz. this brings us to the resin.
polyester resin is the cheapest , vinylester is the other end price and strength wise. and there are several others like gp and iso and in the same way that different glass types are used for specific things so are the resins. some have low shrink, some are high strength, some are better for below waterline, etc.
the thing about the esters is that they only bond to the same type. this is why you usually have a hard time bonding to metton like is used on the bottoms of some of our skis, its a different resin. its not going to stick to epoxy resin either. and before anyone jumps in and says they did it and it works, yes I know cause my cheapazz did it too. but its not gonna last. so if you lay up poly or the repair is made with poly you can bond to that with poly resin. also the ester resins will disolve the binders that hold the chop glass where the epoxy wont. you also should not use it for carbon or kevlar as its not as strong as epoxy. the resin is the binder that holds the glass or carbon together so it makes no sense to use a weak binder on the strong material. so for example if you want to make a mold you can use chop and poly since its not going to be subjected to any forces but just needs to be thick to withstand curing and not flex and also does not matter how heavy it is. but if you want to make a competition freestyle ski you want the least weight which means use the strongest materials so you can use the least amount of layers. you will also orient the glass to make the best use of its strength in a given direction.
epoxy resin will bond to ester resins and just about everything else.
one last thing, with kevlar and other aramids, they dont absorb the epoxy resin like other materials do so you essentially are just encapsulating them in resin. this can lead to issues with delamination which is why its usually best to not mix them with other materials. since regular glass has different strength vs kevlar is kinda like how different metals expand at different rates, so while your fiberglass may bend or stretch and has some give the kevlar might not have the same amount of give and this can cause them to break away from each other.
so to put this all into perspective this is how I use what ive just described.
if Im making a mold I would use chopped glass 1 or 2 layers. starting out I would lay resin down and break small pieces ( torn not cut ) and place in any funky areas first. if you have holes or scrapes you need a filler usually to fill that stuff. im saying if I have a small dip or step, put that down first so the next layer can lay flat over the area. then switch to a heavier material like 1708 or 1700(no chop) and put 3 layers down. you have to be careful not to do too many layers at once. so stop there and if needed when that kicks lay up your next layers. poly resin or something cheap. if its gonna be a mold ill use long term use iso resin.
if I wanted to build a light thin flexible part like say an rc car body, I might go with several layers of 4 oz of 6 oz. w no chop this time. this will hold its shape but bend without breaking a really good amount. id prob use a gp or vinylester resin
if you wanted to build something really strong and light use carbon and epoxy. the best way is vacuum bagged so you can get the proper resin to fiber ratio. thats a whole other topic though.
if I needed to fix a hole like in the top deck where a bilge fitting was, I would use several pieces of 10oz cloth to build it up inside and out and could use an ester resin since thats what its made from. or say something bigger like you shorten the hull I would use epoxy and some thicker material. obviously if you had tight areas you would start with the chop and then switch to the bulky material.
for a tray you can do a lot of ways. lay a single layer of 1208 or 1708 in a big piece like the size of a full sheet of turf, cut out the parts. maybe mock up in carboard first and transfer to the glass, and then use 3 or 4 more layers to bond it all together and into the hull. you can pour foam into the gutted tray and sand it all to the shape you want and then glass over. you can make a wooden mold or whatever material you want and lay a tray up off that and install. ive taken a squarenose that had a cracked tray, cut out the insides of the gunwhales, refoamed and re-layed the tray flat, side to side and then reinstalled the sides which I also thinned down to make it a wide tray and bonded all that back in. that thing will never break again. the owner was a big guy and wanted to make sure of it.
last thing about glass is its awesome! you can mess up- grind it back down and re-glass. Trust me, if I can do this, anyone can do this. Dont be afraid to make mistakes. sometimes they are the best way to learn. I like to call it failing upwards. I might not be able to tell you how to do it right but I can tell you a hundred ways not to do it! and that is equally valuable. oh and be safe, protect yourself, gloves, mask etc all that stuff is cheap when compared to a hospital visit.
Hope my very limited and very basic info helps somebody. Im sure volumes of books could be written on this stuff but I just tried to cover what I use and how I use it. if you have questions post em or shoot me a pm. Im not around like I used to be but im still around.
And please feel free to correct, or add to anything ive posted. you never stop learning, I know Ive learned a lot from others on the site.
