Planning my 1975 -20' Open restore

[flyingfrizzle]

Originally Posted by brian_gem

Thanks flyingfrizzle.
One question I have is about the chopped strandmat..I haven't seen a lot of people using this, where would you recommended using it? Most I've seen are using a 12oz tape, and 1708.

Even tho CSM has little strength compared to 1700 & 1708 it still has it's place. To give you a few examples where I use it sometimes: Say I am patching some holes in a console, I would bevel them back real well and then depending apond the size of the hole and place I would cut a core to go in it or put tape on the back side to back it and hold the lay up from falling threw. I would start with Some csm just to fill the hole in a bit then use 1700 or 1708 to build it up a bit then cap it off with several layers of CSM. When you sand it flush you don't want you last layer to be 1708 for one it will not sand down good and blend well. The stitching will show threw and cause problems plus once you sand threw the strands in 1708 it looses all its strength. What makes it so strong is the long straight fibers, and if you cut them while sanding it flush it weakens it a lot. The last layers of csm on top will sand down easier and will blend in a lot better as well. It also maintains a barrier for the 1700 strands and keeps them from being cut. That's just one example but any where you have a surface that needs aggressive sanding and fairing I always leave a layer or two of csm on top of it so that It gives me something to sand down into. I sometimes even mix in some micro balloons with the resin that I lay the last layer of csm up with and that helps make it a lot easier to sand too. Some guys just use 1708 then skim it will fairing compounds and sand down into the compound to fair out a surface. That works to but its like a car skimmed with bondo, if you get it too thick it will crack badly. The csm will not crack near as bad as straight thickened resin with a additive like micro balloons. The quick fair product and others like it will work well if used right but you cant leave that stuff on there too thick at all unless you want to deal with cracking latter. Glass work is just like car body work almost, I see some people get there glass work real close like a good body man and I see some get there work not as close and skim 5 gallons of quick fair on them then long board sand them and both leaves a good finish but one is better than the other in my opinion. One will remain problem free for much longer. Doing it with the csm may be a little harder to sand but its wet on wet primary bond unlike skimming putty on a cured lay up. The bond is not near as strong. Don't get me wrong, I use Water tight epoxy filler on my hulls but in small amounts and laid on thin. http://www.jamestowndistributors.com...e+Epoxy+Filler

Another area that csm is needed is between the cores if you use say two 3/4 pieces of marine ply to make your transom core. The wood will soak up resin at a high rate and csm lay up is around 50% cloth 50% resin. Several layers of 1700 stacked may be 70% cloth and 30% resin. The high cloth to resin ratio is what makes the lay up stronger along with the long strands of cloth. But when bonding two plys together you want the extra resin so that the wood has it to absorb. I hot coat my wood with thinned resin (thin with styrene) and that helps a lot cause if you skip that step the wood might draw out to much resin out of the csm and leave bare fibers and then you will loose a lot of strength.

CSM is also good for filling voids, if you have a area of old glass that is ruff and not level and true you can use the csm to level it out and fill in the voids. Again that gives you a layer to sand on so you can fair it out true. The 1708 will not fill in voids as well and leave air spots. The csm on the back of the 1708 will fill some but not all.

One other use for CSM is to prevent print threw. When laying up a mold you must lay several layers of csm down first before the biax (1700 or other like woven roven) to prevent the pattern of the cloth from showing threw. The original seacraft potter built hulls have a gel coat and then several layers of csm then woven roven laid up to make the hulls. Well even with the csm barrier you can look down the side of some of the hulls and see the woven roven pattern showing threw. The csm will prevent a lot of this if done right and with out it the pattern would show threw way worse.

Im sure there are lots of other areas that it is needed as well but that is a few examples. I think you should use 1700 or 1708 when possible but sometimes the csm is just better for the situation. Just remember that a standard starch csm can not be used with epoxy. YOu have to use a special csm for epoxy that will work with it. Standard csm has binders in it that the styrene will dissolve where epoxy is not styrene based like polyester or vinyl ester and cant dissolve the binders. Also the 1708 you buy for epoxy must be sowed together or stitched together (stich matt). It cant be bonded together with starch binders either. I use the stich matt but the stitching show threw the lambent bad if you sand down in to them. That's another reason why it don't make a good top layer. Use peal ply when you can to keep lay ups flat and clean. It ant cheep but makes for cleaner looking lay ups. I use a plastic film that comes from an plastic lamination process because I can get it for cheep vs the peel ply but it is worth spending the money on it if you want a smoother surface when you pull it off. Cool thing about the peel ply is you can lay down new layers on the cured glass with out sanding after peeling it off. It leaves a slight texture that the new glass will inhere to well. If you don't use it the surface needs to be ruffed up with 80 or 120 grit to give the resin some tooth to bite into.

I anit no professional but this is what I found out over the years that works best for me. Be glad to help if you got any other questions.