Polyester resin is mixed with a small amount of catalyst. It cures very quickly. Polyester resin is commonly included fiberglass repair kits found at auto parts stores, hardware stores, and marine supply stores. I have been told, by those who know, that polyester resin will allow moisture to penetrate the resin itself. However, many boats are built with polyester resin by the manufacturer.
Epoxy resin is a two part resin that must be mixed at the right proportions. Epoxy resin is an excellent bonding resin. It is very strong. It does not allow moisture to penetrate.
There is an effect called "amine blush" that results from the cure of epoxy resin. If this develops between applications of epoxy resin, then subsequent applications may not adhere properly. Amine blush can be removed by simply washing with soap and water. Epoxy resin is available from a variety of marine suppliers and can be more expensive than polyester resin.
Which resin you choose is up to you. I submit that if you do quality work, that it really will not make a difference. I used epoxy resin in my build.
This is a subject that gets the experts fired up as much as the debate of epoxy or polyester resin. Cloth and mat are very different from each other but act in the same manner. When saturated with resin, this material is what makes fiberglass so strong. Ideally, you should have equal ratio of fiberglass and resin.
Fiberglass mat is made of various strands of fiberglass that are not aligned in any certain pattern. Standard mat is glued together with a glue. Mat used with epoxy resin is actually stitched together. They are not the same! Like cloth, mat is available in various thicknesses. One benefit from the mat is that it is easy to form around odd shapes. It is good as a bonding layer.
Fiberglass cloth is actually woven together like a fabric. Again, it is available in various thicknesses. It is not as easy to form around odd shapes. Some experts say that it produces a stronger fiberglass than mat. Also, it produces a smoother finish.
I often use both in my repairs, even in several layers. For instance, start with mat because it will form better around the repair. Then follow with cloth as the finishing layer. I have seen both included in fiberglass repair kits.
Fiberglass cloth is available in pre cut rolls in various widths from two inches to eight inches. I commonly used two inch, four inch, and eight inch rolls. This allowed me to double the coverage of each application getting good thickness.
Another option for some applications is specialty cloths. One that I used on my floor project was a thick biaxial weave combining cloth and mat in several layers. This allows for rapid build up and gives great strength.
It is a good idea to use several types of fiberglass material, based on the particular application. For instance, I recommend using fiberglass mat to bond stringers to hull; or wood floor to stringers. It is also good for the first two layers of build up. Fiberglass cloth is best for laminating and for the final layer of application. Biaxial is great for rapid build up and extra strength. I used all three in the floor.
There are a number of fillers on the market that when mix with the resin. Some fillers are used just to thicken the resin so that it does not run easily. Others are for smoothing the finish. Some can be used to strengthen the resin and fill gaps or fillet. Not all fillers work with both epoxy resin and polyester resin. The best thing to do is follow the manufacturers recommendations.
When working with either epoxy or polyester resin a common filler I used was aerosil cabosil. This is just a thickening agent. When applying resin on surfaces that are perpendicular to the hull the resin will go with gravity. This filler allows you to thicken the resin to the point that it will not run off so easily.
Another common use for fillers was to fill small gaps and to fillet along joints. For instance, once you laminate the main stringers onto the hull there is a sharp angle at the union. It is recommended that you build a fillet along this joint. The fillet should be at least 1/2 inch thick and at least 1/2 inch tall and wide. This fillet strengthens the bond between the stringer and the hull similar to that of welding a fillet.
Using a filler material maintains the proper proportion of resin and glass (in this case filler). I used West System 403. I learned later that this is made up of cloth like material. A better filler would have been the 404, which is made up of tiny glass fibers. Bottom line here, check the specs for the best product for your particular application.
Another common filler I used was a basic fairing compound. It is possible to use plain talc. I used a filler from US Composites called "fairing compound". This filler is used to help smooth and level the finished surface when working with epoxy. There are other versions that contain tiny glass bubbles. Again, some can be used with both epoxy and polyester, some cannot. But this is one type of filler you will need in a large project.
Check out either the West System or US Composite guides on which fillers to use for your applications.
I used polyester gel cote that I purchased from US Composites. They offer white or neutral gel cote. I mixed blue pigment with the neutral for the blue color in my project. When mixing your own colors it is a good idea to mix it all the first time. The next time you mix may not be an exact match. If you are running low and realize you need more, stop using it. Mix another batch. Then mix the new stuff and the old stuff together to minimize the chances of not getting a nearly perfect match.
Spectrum Colors offers custom colors if you want to match your origional gel cote or choose another color. Still, you need to order enough the first time. I am not saying that two cans of sprectrum Oyster Shell will not match nearly perfectly. Just recommend you plan ahead.
When working with the gel cote from US Composites I used styrene to thin it enough to spray from an automotive paint sprayer. The styrene is available from US Composites.
The last application of gel cote before sanding should include a surfacing wax. The wax is a sanding aid. The wax is available from US Composites. Both the styrene and wax are mixed into the gel cote before you apply it to your project.
I used epoxy resin from US Composites. Their company offered mixing pumps. This eliminates the need for measuring in separate cups then mixing in another container. I believe the West System offers similar pumps. Be careful to get the right pump for the ratio required.
The polyester resin is mixed with small amounts of MEKP or similar agent. Only a few drops are required for small amounts of resin. Most suppliers carry a variety of tiny measuring cups that are pefectly suited for measuring small amounts of liquid. These cups can be purchased in bulk.
Plastic measuring containers are available in many sizes. I commonly used the quart size containers for both resin and gel cote. Again, I suggest buying in bulk.
Tongue depressors, small and large wooden stir sticks can be purchased in bulk. You will go through a lot of them. They are inexpensive, so buy a bunch.
You should consider how much of these items you plan to use during your project before you start. Everytime I had to order more supplies I lost a lot of money on shipping and it set me back in my progress.
Fiberglass resin can be applied with a paint brush or roller. It can even be sprayed with the right setup. I always used brushes. I chose to use inexpensive brushes that I would discard after each use. Some people prefer to use higher quality brushes and clean them after each use. It is your decision. If you go with the cheap brushes, like I did, they can be purchased in bulk. I suggest that you buy a bunch of them. I primarily used four inch brushes.
Once you apply resin to fiberglass mat or cloth you have to roll out the air bubbles. Air bubbles in the finished product are a bad thing. I bought fiberglass rollers. You can make them yourself using washers by alternating larger and smaller diameter washers on a trim paint roller. I cleaned the roller after every use, but still went through three rollers in the six months I worked on my boat. They are available at most of the marine suppliers I mentioned earlier.
The fiberglass material can easily be cut using utility shears. I found a set of shears that came apart at the handle to facilitate ease of cleaning. Once fiberglass resin has cured on the material you can use a utility knife to trim the excess from edges of your work. Shears don't work so good on the cured resin and glass.
I frequently used a carpenters level and dial calipers for measuring the height of the stringers. I used a hand plane to trim material off the stringers to adjust that height. I also used the hand plane to bevel the edge of the stringers where they fit against the hull. There are some useful tools to have on hand.