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Resins (Plastics) Polyester and epoxy

Plastic resins are used for a variety of purposes during modern boatbuilding. (Modern, because these plastic resins did not exist during the wooden boat era). There are an abundance of types of plastic resins, but for boat building I found that I used only two types of plastic resins, polyester and epoxy. These plastic resins have considerably different characteristics and are used for different purposes. I only use polyester for fiberglass layup and making a putty for use with polyester fiberglass layup. The reason is that epoxy will adhere well to polyester, but polyester will not adhere well to epoxy. In addition, epoxy is a glue as well as just a plastic resin, and tends to adhere to virtually all materials. (I have read that it will not adhere to pressure treated wood due to the incompatibility of the various chemicals).


Fiberglass Layup

Fiberglass Layup needs to be divided into fiberglass with polyester resin and fiberglass with epoxy resin. With all the reading that I did about fiberglass, mainly Ken Hankinson’s “Fiberglass for the Amateur”, I came to several conclusions. One of the most important was that polyester resin is fine for making parts, including the hull but not good for joining parts, even parts that were laid up with polyester resin. There are some timing issues, but basically, polyester resin will bond well with itself as long as it is not fully cured. Once it is fully cured, it won’t bond even to itself. Polyester resin will not bond well at all to epoxy, but epoxy will bond with polyester. An easy way to remember this is that polyester resin is not a glue but epoxy is. With certain restrictions, epoxy can also be used as a layup resin.

Layup with polyester


Basic Rules

Use biaxial cloth with sewn on mat.
Lay the material with the mat against the mold or previous layers. The mat provides a good bond between the layers because the fibers will move around and tend to hold a lot of resin. This reduces the air bubbles. In addition, using the biaxial/mat combination allows the use of squeeges and bubble rollers on the sewn biaxial without causing the fibers to clump and pull. The older approach is to use a layer of mat with a layer of roving on top. This is a little less expensive but is much more difficult due to the two layers being independent and able to move relative to each other. In addition, the roving strands will move relative to each other. This makes  wetting out the two independent layers simutaneously considerably more difficult. Note that the mat/roving approach is a good one, but requires considerably more experience and skill.

Use new resin
Polyester resin is a strange product in that the minute it is created it starts to cure. (Unlike epoxy that requires two parts to start the chemical cure process). This means that the longer the resin sits on the shelf the more it cures, even before adding the catalyst. The more the resin cures the more viscous it becomes and is much harder to use for layup work. (I do not know the cure time without catalyst, probably measured in decades, but it will cure on its own eventually). The newer the resin the easier to use. The catalyst speeds up the process the same regardless of previous shelf life. With new resin it is also much easier to mix the catalyst. Lots of reasons to ensure that you have new (fresh) resin. Fresh resin has roughly the consistancy of water. Not very viscous. In fact, to use resin on a vertical surface, there need to be additives to increase the viscosity. 
With new “high quality” resin, the resin will noticeably change color when the catalyst is added so you can tell, with experience, that you have added the right amount of catalyst.

Wetting out fiberglass is not rocket science, but you have to move quickly. The resin must be spread quickly and evenly and the air pockets need to be worked out of the glass. There were four basic tools I used for wetting out fiberglass: rollers, squeegies, chip brushes, and bubble busters. I would typically roll the resin onto the material, use the squeegie to move excess resin to where it was needed, use the chip brushes in the odd spots (where there was a curve etc) and use the bubble buster where air was trapped under the cloth and resin. I bought the chip brushes in bulk from anywhere I could, the fiberglass supplier, specialty tool places, industrial products (Grainger etc.). I bought the squeegies from Home Depot. They have a large selection of sizes, you can make your own handles, and they have replaceable rubber pieces. I made handles of various lengths. The most useful handle length was about 3 feet, it extended your reach but was easy to maneuver.

Hull Layup (Bulk Layup)


Bulk layup requires many layers over a large surface.

  1. Do not bite off more than you can chew. Do not try to do too much at one time
  2. Multiple layers, unrolling as you go is possible, but much more difficult than you might think. I gave up on trying to do this.
  3. For a large piece of Glass, keep a wet edge.  Serially catalyse containers of resin. I used quarts, which was about half of the amount  I could wet out before it started to “kick”. Catalyse a quart, use that quart to wet out as much as you can, starting from an end of the glass. When you are about ¾ complete with that quart, catalyse the next quart. Keep moving in the same direction wetting out the glass. About the time you have used up the second quart, the first quart will have “kicked” but you still have the wet edge that was wetted out using the second quart. Keep going in the same fashion, catalysing the subsequent quart of resin when you are about ¾ complete with the previous quart of resin.
  4. Butt the fiberglass sections for a given layer that you expect to do in a day.
  5. Overlap the joints in subsequent layers.
  6. Measure out as many quarts as you think you will need prior to starting. Maybe one or two extras. Do not catalyse until ready to use.
  7. Purchase the special squeeze bottles with the measuring device on top for catalysing
  8. Calculate, then test, the amount of catalyst per quart prior to using a given batch of resin and catalyst.
  9. If it is a really hot day, you can ice down the resin  (already poured into the quart containers) in a cooler prior to using. This will give you an extra few minutes of resin work time.
  10. Buy or save huge numbers of quart size plastic buckets for the job
  11. Let the residual resin in the buckets harden, then try to remove the resin by flexing and tapping the bucket so that you can re-use them. You should be able to use a bucket about 3 times before it gets so crudded up that you cannot remove the residual resin without cracking the bucket. Do not reuse the bucket unless you can get it free of any cured resin that may contaminate the next batch.
  12. Buy lots of chemical resistant gloves for doing the work (found in most fiberglass supply houses at a reasonable price)
  13. The chemical resistant gloves can be reused multiple times until they crack. Stretching the glove after the resin has cured, will free most of the resin from the gloves.
  14. Be very careful with the catalyst. It is extremely dangerous. Do not let it touch your skin, get in your eye, breath the fumes etc. The integral measuring/storage bottle really helps. Wear gloves.
  15. Buy your fiberglass by the roll/pound. Much less expensive.
  16. Buy the resin by the drum. Cheaper by a large margin. You can buy a resin pump to use with the drum. You should be able to buy the pump from the resin supplier.
  17. If you are doing a lot of layup, consider buying acetone by the drum. Acetone seems to be the best solvent for polyester resin.
  18. Grind the surface of each layer prior to laying the next layer. (Brutally hard, but in my mind, absolutely necessary)
  19. Never, never, try to lay a single piece of glass with two inside corners.

When I laid up my hull, Iwas laying up inside an existing hull. I would find a way to secure multiple pieces of cloth around the hull and cut them so they would tape down and match up at the joints. I would use pieces the width of the roll (about 50 inches) and about ten feet long. This gave me natural stopping points if things got ugly. Grind in the morning and layup in the afternoon was a typical day.

I ground the fiberglass between each layer. There were two reasons for this. One was that there was no way I could get any layer smooth enough to take the next layer. There was always a few spots where the glass would be sticking up. This usually happened where the glass had edges. The edges were either the ends of pieces that were butt jointed together, or where the glass needed to be cut to fit an odd shape. 90 degree biaxial works a lot like plywood or paper. It is reasonably dimensionally stable which means that it only wants to take on shapes that are conic sections. There was some leeway in this, but the complex shape of a sailboat hull required a lot of cuts in the fiberglass to allow it to lay down properly. These ends would invariably have an unevenness that needed to be smoothed out. The other reason was that if more that two or three hours had elapsed between layers, the polyester resin would pick up contaminates (mainly moisture) from the air, and this would inhibit the adherence of the next layer. (occasionally I got lucky and was able to lay more than one layer prior to a full grinding. The uneven areas always needed to be ground smooth)

Laminating table Layup


If any “flat” pieces are required, a laminating table is a great boon. It facilitates complete layup of pieces without the delay and grinding. It also can be made to be a convienient height for ease of use. I wanted a 90 degree angle on some of my pieces so I made a laminating table with two laminating surfaces, the top (4 ft by 10 ft) and down the front (4 inches by 10 feet). I built the laminating table from 2X4s with a ½ inch plywood covering. I then went to a cabinet shop and bought their left over (ugly) formica. I used contact cement to attach the formica to the plywood. In the corner I use ¾ inch quarter round trim. I waxed the whole thing and was ready to go.

Laminating on the laminating table uses the same approach as larger scale laminating, except that you are building up layers of material. The technique is to estimate the amount of resin necessary for each layer, then pour enough buckets of resin (un-catalysed) to complete the layup. (Max of 6 layers). Pour or roll resin on the table, lay down the first layer of material, wet out the material, lay down the next layer, wet out, etc. Keep going until out of the first bucket of resin. Catalyse the next bucket of resin just prior to needing it, then continue the process until up to 6 layers of fiberglass are complete.

The six layer limit has to due with temperature. If you try to lay too many layers of glass at the same time, the resin will “cook” and discolor and weaken, and if too thick, could actually get hot enough to catch fire!
The reason for this is that polyester resin, as it is curing is “exothermic” (gives off heat), the more layers the more heat, with the surface area the same to disburse the heat. (Each layer generates heat, but only the top and bottom layer give off heat to the air or table. More heat in, same heat out, temperature goes up). Six layers is not an absolute number; however, I have found that six layers never gave me a problem, and I ended up with about a 3/8 thick sheet of fiberglass reinforced plastic that was a little larger than the size I wanted. The pieces were then cut to fit and tabbed in place.

The reason for putting resin on the table prior to setting down the first layer of material is to wet out the material from both sides, speeding the wetting out process. This is not necessary on subsequent layers due to the fact that there is usually a significant amount of exess resin in the wetting out of the previous layer. (If not, add resin to the wetted out layer prior to laying the next layer of material). Typically the last layer will wet out from the bottom with very little additional resin applied to the top of the layer.

Piece part layup

To make fiberglass tanks and other items for the boat, I used what I call my “piece part” technique. The traditional way to make fiberglass parts for a boat (read production) is to build a mold, lay up the glass, then discard the mold. I thought that this was additional work, since one off construction usually implies using the mold only once. The technique that I used was to make the mold from sheets of glass that I had laid up, glueing the pieces together with polyester resin fill, then finishing the layup around the original mold, with the mold being part of the finished produce.

The fiberglass sheets were one or two layers of  composit biaxial fiberglass material laid up into sheets on the laminating table. I would build a frame out of scrap lumber, bend and clamp the sheets to the frame. The next step was to make a fillet out of polyester resin on the inside corners and sand the outside corners to get a reasonable radius that could be glassed over. At this point, laminate the corners, which holds the structure together, remove the frame and layup over the fiberglass mold to the thickness required.

This process saves a little time and money, but you still have the issue of providing the necessary finish. Fairing and finishing fiberglass is another topic.

Fiberglass Laminating using Epoxy

Limitations of my use of epoxy


The first thing I need to say here is that I did not perform any major layup with epoxy resin. I used epoxy resin/glass layup for a protective coating over the deck and cabin, which were made of strip planked fir. This entailed only two layers of epoxy/glass. The other uses for epoxy were tabbing pieces together. These pieces could be wood to fiberglass (polyester resin pieces), fiberglass to fiberglass etc. In addition I used four layers of glass reinforced epoxy to make non structural “ribs” to which I attached the “ceiling”.  (Ceiling on a boat is the covering of the hull that performs the function of the inside sheet rock for a house).

That being said, here is what I found using epoxy as the “plastic” in fiberglass reinforced plastic.

Epoxy vs. Polyester


There are two major differences in the layup using Epoxy vs Polyester. One big difference is that the binders in fiberglass mat do not dissolve in epoxy. This means that for epoxy, a woven or sewn material without mat is required. I typically used a biaxial sewn material for epoxy layups. The other big difference is the difference in the viscosity and cure times of the resins. Polyester is very watery and kicks very quickly. Epoxy is viscous, doesn’t “kick” in the same sense and cures slowly. The working time of epoxy for layup is about double the time for polyester, but where polyester “kicks” and becomes hard at the end of the working time, epoxy just gets gooeyer and gooeyer until it will not wet out the material. Based on the epoxy used and the conditions (temperature) the working time of epoxy varies dramatically. The general purpose epoxy that I used, typically had about a 15 minute working time in 90 degree weather.
Layup using epoxy requires a great deal more working because of the initial viscosity. It is interesting that working the epoxy helped only slightly in the wetting out. The trick seems to be to get the epoxy on quickly so that it will soak into the material prior to curing to the point where the viscosity is to high to allow the epoxy to soak into the material. One advantage is that if the epoxy on one wetted out piece is too viscous to soak into the next layer, then you can externally wet out the next layer and place it onto the existing layer and the epoxy will bond with the already curing epoxy. At this point you should have wet epoxy to which you can add additional layers.

Safety and Clean up


Polyester resin has a lot of volitile, toxic, solvents that evaporate into the air. Be sure that you have very good ventilation or an external air supply. Epoxy resins are pretty benign by themselves; however, the solvents for epoxies are also toxic and volitile. The main clean up for polyester resin is acetone. Be careful with this stuff. Remember, it dissolves paint! Also lots of fumes. The clean up for epoxy is the “solvent/reducer” . Very toxic and volitile. Acetone can also be used for epoxy.

Epoxies have a couple of properties that are significantly different than polyester. People can have or develop an allergy to epoxies. It is my understanding that,  like poison ivy, you may not be allergic but continuing exposer can bring on an allergic reaction. Minimize exposure to epoxies by wearing disposable gloves and a respirator (the filter kind, not the dust mask kind). A neat thing about epoxies is that when they are not yet cured, they can be cleaned up with vinegar. Yep, plain white vinegar.


Other uses for Epoxy and Polyester Resins

Polyester and Epoxy putties



Both polyester putty and epoxy putty are made by adding various thickeners to the resin. The two main additives are silica and microballoons. The putty made with microballoons should only be used above the waterline.

It is interesting to note that the addition of silica to either resin does not appreciably change the volume; whereas, the addition of microballoons does significantly change the volume.


Polyester fill

Polyester fill is used when building various parts (the hull included) using polyester resin and glass layup. The fill is used to make fillets, and to smooth rough edges so that the next layer of glass will not have built in air pockets. I made polyester fill with standard polyester laminating resin and adding only silica. The way I was taught was to mix silica (often sold as Cabosil or Aerosil) with the resin prior to catalysing the resin. This will make a resin paste that will not “kick” until the catalyst is added. You take the paste and put it on some sort of board (same idea as a mortar board). When it is on the board, make a little vocano (remember mashed potatoe volcanoes for holding gravy? This is the same idea for holding the catalyst.) put plenty of catalyst in the volcano, usually two to three times the amount normally used for that amount of resin, stir in the catalyst until it becomes a uniformly new color (color depends on the resin, but all of the resins I have used exibited a noticable color change). At this point, use a putty knife, ( I usually used 4 to 6 inch drywall knives) and putty away until you have the surface you are looking for. Watch out for the “kick”. Until you have experimented, it can catch you by surprise.

I never use polyester resin putty for gluing, or any purpose other than filling freshly constructed polyester/glass parts.

Polyester fairing compound


My advice here is simply do not use it. Not the manufactured kind, nor the homemade kind.

Epoxy fill


Epoxy fills can be used for  several purpses and usually require different additives for the various uses. The general uses of epoxy fill are: bonding, below waterline fairing, above waterline fairing, fillets, gap filling glue. Building the “Theodore Robert” I used two types of epoxy and epoxy fill. In the earlier stages of building I typically used “West System” epoxy. I had read a great deal about it and they have a great marketing machine. The advantage of the West System is its flexibility, from different hardeners for different cure times to many different additives for various purposes. When using relatively small amounts of epoxy, this made life easier, but there were two problems, cost and performance. The cost of a gallon of “West System” epoxy is about 2 ½ times the cost of a general purpose laminating epoxy. The performance of the “Fast Curing” version of the “West System” was not very good, (weak bonding and weak strength). As construction continued, and I was laminating glass with epoxy the price of “West System” became prohibitive and I started using general purpose laminating epoxy.

Epoxy fill is used where any filling is needed other than on polyester layups. You cannot lay up  polyester resin/glass over epoxy so do not try. Once you go to epoxy, you have to stay with epoxy. The proceedure for this is somewhat different due to the different characteristics of epoxy and polyester resins. For epoxy, you mix the two parts thoroughly before adding the cabosil (silica). Add silica until you reach the consistancy that you want. Apply the mixture quickly. It turns out that the paste that you make with epoxy will hold its form well unless you work it too much, then the stuff starts to ooze. It will also start to ooze if you do not apply it during the first 15 minutes. I do not know why this is (I can guess) but if you apply the putty and leave it alone it will hold its shape until it cures. If you keep working it, it will sag and make a mess that you have to grind off! This epoxy putty cures to a very hard, waterproof fill that is extremely hard to sand. This works well for bonding or for fill that is below the waterline.

The fill can also be used to glue irregular surfaces. In fact, all of my glue joints followed the process of painting the surfaces with epoxy (before additives) making some fill (using silica) and applying the fill on top of one or the other surface, then holding the two surfaces together in some fashion, clamps, screws etc.
There are other additives that supposedly give different characteristics, but for general purpose filling, above and below the water line I have found silica gives good, strong results.

Epoxy fairing compound


Fairing compound is typically made from epoxy and uses a combination of silica and microballoons and is only used above the waterline. The microballoons are the key ingredient here. To make the fairing compound, mix the epoxy components first, then start adding microballoons. Typically I add enough microballoons to approximately double the volume. At this point the mixture is something like a thick pudding. This is too watery to be a good fairing compound, so I add silica until I get the consistancy that I want. It turns out that if you use microballoons exclusively, you can reach the approximate consistancy of putty, but then the mixture will not stick to things well.

This fairing compound is relatively easy to sand and leaves a very good paintable surface after proper sanding. I did find that some of the compound that I made would have tiny pinholes in it after drying. It appeared that this happened when I had used too much of the microballoons and not enough silica.

I used this fairing compound on the topsides and to cover the weave on the epoxy/glass layup. The alternative to covering the weave with fill, is to sand it smooth. Sanding the epoxy/glass surface directly caused a considerable loss of the fiberglass material by the time it was smooth enough to paint. Using fairing compound to fill in the weave, maintained the integrity of the glass fibers so that the strength was not diminished.

Helpful Hints:


1. Vinegar cleans up wet epoxy.
2. Denatured alcohol allows you to smooth epoxy that has gelled but not hardened. It also works for clean up of wet epoxy. Got this one from "Chesapeake Light Craft".



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