Introduction: Fiberglassing Tools and Tricks

Picture of Fiberglassing Tools and Tricks

If you need to do some fiberglassing with cloth and epoxy resin, this Instructable can save you some mess and grief. Manufacturers of fiberglassing and epoxy materials offer booklets that can provide you with instructions for success. However, through my own experience I have developed helpful tools and tricks you won't find in the books. Intro photo is of solid 14-foot boat hull on the right and its twin on the left that I fiberglassed and painted, one my several projects that taught me these lessons.

Step 1: Specialized Tools

Picture of Specialized Tools

The yellow plastic spreaders are recommended for pushing the epoxy into the glass cloth, however they are hard to grip when your gloved hands are wet with epoxy. I used two screws and nuts to attach the plastic handle of a defunct disposable foam paintbrush to each spreader. I prefer the rubber spreader for big jobs and added a handle made from a length of broomhandle that I slotted, and two strips of sheetmetal to add side-to-side stiffness. Both kinds of spreaders are available where epoxy products are sold and by mail order.
The shape of the scissors enable them to easily cut fabric that is flat on a table, and they have a palm bar rather than the usual cumbersome thumbhole. The scissors have a spring-open feature, are Fiskars item 9911, Wal-Mart, $14.
I use waterless handcleaner, such as Goop brand, and a stiff brush to clean these tools before the epoxy can set, and after many uses those in the photo are still clean and not sticky.

Step 2: Mix Small Epoxy Batches

Picture of Mix Small Epoxy Batches

I found a way to mix small batches of epoxy resin and its hardener, in proper proportions, even if the liquids are in large containers. A little effort in advance will make the job easier. Mixing epoxy in graduated containers is costly, and measuring the liquids in a graduated container then pouring together into a small mixing cup can lead to inexact proportions. First, get sleeves of a few sizes of clear or translucent plastic or paper cups (not waxed!) that will hold the different amounts of mixed material you think you might need, with enough room free at the top of the cup to allow for stirring. Using the following notes, make a reusable "calibration half-cup" for each size cup and for each size batch within that cup size.
Note the half-cup has its upper rim cut off so it will nest perfectly onto the side of a new cup. It is marked "2R:1H," to denote that this half-cup is to be used with epoxies that call for a ratio of "2 parts Resin to 1 part Hardener."

Step 3: Make and Calibrate the Half-cup

Picture of Make and Calibrate the Half-cup

Photo shows how "calibrated half-cup" is put onto the side of a cup that will be used for mixing a small batch, and the calibration marks transferred to the new cup.
For each cup size, take a new cup and put WATER into it that is about one-third the amount of the final batch of mixed epoxy, and make a mark at that level on the outside of the cup. Pour all that water into a clean cup and mark its level on the outside and label that mark as "H" for hardener. Put water into the first cup again, to its marked level, pour that water into the second cup on top of the water already in it, then repeat. Mark the top of the water level on the outside of the second cup and label it "R+H" for resin plus hardener.
Repeat the procedure if you want to calibrate the second cup for a larger epoxy batch in that same cup size, putting the level marks on the opposite size of the cup. Cut the calibration cup in half vertically, and keep it with your stash of that size cup.

Step 4: Small Batch Cup Ready to Use

Picture of Small Batch Cup Ready to Use

Small mix cup has calibration marks (fattened to aid in photography) transferred to it from the calibration half-cup, and the marks will be visible through the translucent sides of the cup. Viewing into the cup, carefully pour hardener into the cup to the first line, then carefully pour resin up to the top mark, and you will have exact mix ratio. Mix and use from that one cup, then discard it.

Step 5: Foam Clamps

Picture of Foam Clamps

Vaccuum bagging is great for clamping fiberglass tightly over a shape and removing air bubbles, but it is too complicated and costly a procedure for occasional use. For smashing wetted cloth tight to the sharp bow of a hull, I made clamps by backing up foot-square blocks of 4-inch thick foam rubber with squares of 3/8-inch thick plywood and wooden sticks. Before use, I wrap plastic wrap around each foam clamp and tape it to the wood backside. Two slide clamps squeeze the cloth and conform it to the hull mold's shape. Rubber foam can be cleanly cut using straight edge guides and a hotwire cutter. Click on link to see album "Cutting Rigid Plastic Foam," click on "READ FIRST" instructions for reading and printing fine text.

Step 6: Squeeze When You Can't Vaccuum Bag

Picture of Squeeze When You Can't Vaccuum Bag

Wet fiberglass cloth can be made to wrap over a sharp edge, but it will lift up off the edge before the epoxy sets unless it is clamped in place. Here is the full-body version of the foam block clamp. I used a sheet of white upholstery foam, 1-inch thick, to press the wet cloth around the bottom of the hull (facing up in this photo). I pre-cut strips of a stretchy 5-inch wide colored foam, sold in rolls at home centers as a barrier between the top of a house's foundation and the house itself, marked each strip in sequence. Once the fiberglass was wetted out, I smoothed a precut strip of thin clear plastic tarp over the wet fabric as a mold release, placed the white foam sheet over that, then stretched each strip of the colored foam and taped its ends together with duct tape.

Step 7: The Mummy Unwrapped

Picture of The Mummy Unwrapped

The hull after the foam has been unwrapped. The fiberglass conforms so well to the shape, even where it is folded double across the front edge of the bow, that it is virtually invisible.

Step 8: Deck Gets Fiberglassed

Picture of Deck Gets Fiberglassed

Once cloth is wetted onto the deck and the edges trimmed, thin plastic sheet is smoothed over it, pulled down and taped to keep the cloth wrapped over the sharp edge of the deck until the epoxy sets.
These hints come from an article I had published in the 01 Aug 1998 issue of MESSING ABOUT IN BOATS magazine. It contains other information that does not fit the step-by-step format of Instructables. Click link to an album that contains the article, click "READ FIRST" instructions for reading and printing fine text.
To see my related projects, enter unclesam into the home page search box, scroll and advance pages to view them all.


BuksV (author)2015-10-31

Ordinary washing machine powder and water works the best for resin cleanup

Ben DangerH (author)BuksV2016-12-31


AlexH35 (author)2016-01-18

Low cost weigh scales are the best for accurate proportions of epoxy as the specific gravities of the two components are very close. Using a .1g accurate scale (max 1kg) for small amounts of 4/1 epoxy and I can reliably make 5 grams of epoxy. I have another set of scales that does 5kg max. The 1gram scales cost $ 15 and the 5gk $ 25. Well worth the investment for easy and accurate mixing.

Ben DangerH (author)AlexH352016-12-31

thank you for the tips :)

thordruid (author)2014-04-01

Hello, we are a small bicycle company, not for profit organisation. for the 20 years jubilee critical mass ride in London, we would like to make a sound system trailor from fiber glass, using a roof top box of a car as a base. We would like to make a miniatur version of the AN 79 or AN 225 aircraft. Could you be so kind to assist us with tips and tricks and how we should go about?? we have never done this job before and we have a 3 weeks to work on the project to completion.

trebuchet03 (author)2007-06-13

Nice work :) Acetone also works well for cleaning those spreaders ;) And I love the sperader handle idea, I'll have to remember that for the future :) For mixing epoxy resin... I use pumps on the resin/hardener bottles and double check in a graduated bucket thingers. They are more expensive than say cups :p But, just let the little bit of resin in there cure, and you can pop out the old resin and re-use the bucket ;)

Poppy_Ann (author)trebuchet032009-05-02

to clean epoxy you can use vinegar which is better for the skin than acetone, also it does not melt styrofome.

pfred2 (author)Poppy_Ann2013-02-10

I won't even open up a can of fiberglass resin unless I have some acetone around.

Ellen the Generous (author)2012-12-19

Great Instructable! You have a lot of really helpful tips here! The one thing that I found confusing was step 5, here. I didn't quite get that you were running us through the process of glassing the hull that you showed in the first image until I got to step 8. That made steps 5 through 7 really hard to follow since I did not know exactly what you were trying to show. If the photos were at more of a downward-looking angle, I would understand that you were working on a hull. Remember, many of your readers will be completely unfamiliar with the content.

With that said, I have an actual question: When you get to this clamping stage, how do you get the downward pressure to pull the cloth over the tip of the hull? Obviously the foam clamp provides significant side to side pressure, but is it that the foam can compress and 'hug' the form so tightly that it provides uniform pressure in all directions?

Ellen, glad to hear of your interest in this project, also sorry that you, among the more than 45,000 people to view this project to date, were the only one who reported being unable to follow the photo sequence. In answer to your real question, this particular foam product is very tough but also slightly stretchy. Before applying the cloth and epoxy, I cut strips of foam to length for fit around the hull while being stretched by hand, with length for a little overlap of the ends underneath, and numbered them in sequence. Once the cloth and epoxy were applied, and still wet, I stretched each strip of foam, one at a time, by hand, around its proper location on the hull, overlapped the ends underneath and taped each strip in its stretched condition, using duck tape. I cut a piece of duck tape and applied it to one end of each foam strip (the end that would overlap over the top of the other end) in advance, before the cloth and epoxy were applied to the hull. The flat deck of the hull was dry while the upturned bottom was being fiberglassed. The wet cloth was trimmed so about two inches hung down past the edges of the flat deck, and the stretchy foam strips were able to bend that overhanging cloth over and under onto the edges of the flat deck and press it down onto the deck until the epoxy set.
Hope this answers your question, Uncle Sam

fireheadfirehead (author)2008-07-07

I am big sailer and im working on restoring a sunfish deck and top. I am tacking off all the rigging and adding a small motor do you happen to know what kind of paint, nonskid, and seaent to use?

Head, I am not familiar with sunfish construction. You might perform Internet search for sunfish owner groups, try to find repair instructions that hopefully would have been included along with each new boat sold. U.S.

pinkishfloydus (author)unclesam2009-03-19

do have any specific advice on how to do a 10 foot kayak? I was thinking about using some PVC to construct the frame, then sandwich that with cardboard, then spraying foam inside, waiting for it to dry, then by using a hand planer I would tighten up the body and give it a better shape, then fiberglassing the finished product. It is for a friendly instructable comp. between me and another one of us. I have to win. Any advice can be sent to me via e-mail @ I apreciate any words.

LynxSys (author)pinkishfloydus2012-12-06

There's a lot of information floating around on how to do kayaks. The easiest thing to do would be "stitch and glue" construction using sheets of plywood that you sheathe in fiberglass. If you want to produce a boat with compound curves, you're looking at wood-strip construction (often called cedar strip), which is also sheathed in fiberglass inside and out, creating a very strong "sandwich core" material. I'm getting ready to build a cedar-strip sea-kayak of my own design, and so I've done a lot of research on these methods recently. I also have a few years of experience repairing commercially produced kayaks, so drop me a message with any questions that Google can't answer. I hope this gets you off to a good start!

junkerjp (author)2012-12-05

Awesome tricks man! I will be using them.

adam.bonahmcarter (author)2011-02-26

Nice one
Another good way to measure quantities is to mark the levels on a stirring stick. As long as the same type of cup is used the stick can be used every time to measure and then stir. A scrape off of any extra mix on the side of the cup, wipe with paper towel and then good to be used again.

JesusFreke (author)2010-05-13

A great (although more expensive) way I've found to mix up small batches of epoxy is to buy a small precise scale and mix the components by weight. You can call up US Composites and get the ratios by weight for their epoxies - they will be slightly different than the ratios by volume, due to the differing densities of the resin and hardener.

I don't recall the scale I had bought offhand, but it was a 500g scale with .01g precision for around $60 (or was it $40? I don't remember). You could find a .1g scale for quite a bit less - probably around $20, and would likely work just fine. The .01g precision is nice to have though, I would recommend it if you can afford it.

Right, excactly. I go through about a gallon of Klear Koat a month and having a scale is a must.

rimar2000 (author)2009-09-10

Thanks for these tips & tricks, they are very useful.

dk-info (author)2009-08-21

Cool tricks of the trade. How do you cut fiberglass cloth so it doesn't fray and stick to everything? Also, which plastic cups can you safely use with polyester epoxy? I am having a difficult time finding plain old paper cups.

unclesam (author)dk-info2009-08-24

dk-info, an even better way to prevent cut edges of fiberglas cloth from fraying. Run wide masking tape along your entire intended cut line. Cut down the middle of the tape, lengthwise, leaving half on your working piece of cloth and the other half on the remainder of your cloth supply. Allow enough excess so the tape will be cut off the edges of your project. The tape will also keep your remainder supply from fraying when you put it away. U.S.

unclesam (author)dk-info2009-08-23

dk-info, another thought about keeping fiberglas cloth from fraying after it is cut to size. I fold short tabs of narrow masking tape over the edges that would tend to fray, all along the length of the cut edge, right after each edge is cut. The tabs are on the overhang that will eventually be cut off. U.S.

unclesam (author)dk-info2009-08-22

dk-info, I try to minimize fraying of fiberglas cloth by having everything else in place before I cut it. I cut on large table next to the project, move the cloth directly onto the project with minimal handling. I cut the cloth larger in the dimension that the cloth will be cut, so it overhangs the project, and fraying will not affect the finished project, then I cut off the overhang after applying epoxy. Re cups. Polyester resin is not epoxy, and I do not use it. Epoxy does not contain any solvents, so does not melt plastic cups, unless it gets hot from curing in the cup. Polyester has very volatile solvents, attacks most plastics. You can find semi-flexible reusable cups in catalogs of epoxy manufacturers and vendors. Once the epoxy hardens, you flex the cup and pop out the contents. You would want to check to see if the cups are made of material that will not be attacked by polyester resin. You might want to check boat repair materials in boat supply stores or marinas, which usually sell polyester resins, might also offer suitable cups. Auto parts stores also sell polyester resin for car repair, might also sell mixing cups. Unclesam

singleprint (author)2008-09-09

I sculpted a 6 foot helicopter body out of styrofoam. Now U need to make a mold of it to reproduce some. However, gelcoat and fiberglass resin will eat the styrofoam. Any solutions would be appreciated.

unclesam (author)singleprint2008-09-10

Singleprint, your question is something of an enigma, since this Instructable shows me wrapping fiberglass and resin around styrofofoam molds, which I could hardly do if the resin dissolved the styrofoam. See also my instructable "Foamboat Construction," that shows more detail of fiberglassing over foam (you can simply enter "unclesam" in the Instructables search box to get list of all my postings). Epoxy resin contains no solvents so will not eat styrofoam. There are other materials used for fiberglassing, such as used in auto body repair, etc., polyurethanes, but they contain very nasty solvents that will definitely dissolve styrofoam, and they are not epoxy. I am not familiar with the chemistry of some materials referred to as gelcoats, but the same effect can be achieved by first coating the styrofoam shape with a coat of epoxy (tinted if you like), allowing it to set up, then applying layers of epoxy and cloth over that. Boating gear stores and boat repair yards usually sell small bottles of coloring agents suitable for epoxy. You will need to come up with some suitable mold release to detach your styrofoam body from the epoxy and fiberglass crust, or you will need to carve the bulk of the styrofoam out and then clean out the remainder using solvent. If you perform a search within Instructables under "fiberglassing," you might find mention of suitable mold release to use over foam (all my projects left the mold in place). The thin, clear plastic sheet sold as painter's tarps can be stretched and shaped over the mold as a mold release, because cured epoxy will not stick to it. It may be difficult to get the sheet to conform to your shape without wrinkles and gaps, though. I hope some of this helps and that you will post your project as an Instructable. Uncle Sam

singleprint (author)unclesam2008-09-10

OK thanks. 5 years ago I measured and proportioned a 69 Camero down to 1/2 size. Carved out of styrofoam. To perfect the surface, covered same with thin coat of drywall plaster and easily sanded result Then to further correct any imperfections, used sandable auto primer and red putty. Made the mold (plug} and dug out the styrofoam. Produced 5 fiberglass cars from that. This time I did the same method, this time making a 2 foot long helicopter, and would like to make a 6 foot helicopter, but the method I been using seems like way too much work and mess, there must be a better way! Do you have experience in making molds starting with a styrofoam plug?

unclesam (author)singleprint2008-09-11

singleprint, I have no experience a making mold from a foam plug, but it also seemed to me that your method was a lot of work, depending on how many copies of the final object you were making. One idea would be to make a shallow box (coffin) big enough to hold one side half of your foam helo plug, and a second for the other side. You would produce two fiberglass halves of your helo, fasten the two together at a seam. You could fill each box with some incompressible material, could be rigid foam, then crudely carve out enough to leave a cavity that was an inch or two larger, all around, than half the helo plug. You would coat the foam plug with a mold release material, a smooth coat, such as silicon paste car wax, slather the plug half and mold half cavity with a fluid molding material, and then partially fill the cavity with it. Next, squish one side-half of your helo plug into the cavity and fasten it so it cannot float up or move, until the liquid sets up. You would need to provide some provision for the excess molding fluid to squeeze out and flow over the top edge of the box. The best mold fluid is latex rubber made for that purpose, but it would take a lot for the large helo mold, could be costly. Plaster of Paris is a lot cheaper, ought to work, but you would need to work fast and use care to keep bubbles from forming on the surface of the plug. Bubbles could be filled and hand shaped with more plaster once the foam plug is removed. Another way to create the mold, in same kind of shallow box, is to put some kind of mold release on your plug and fasten it securely so one half side is in the box, the box empty in this case. Mix 2-part foam, the kind that has minimal expansion and which will set rigid (there are many kinds) and pour or squirt it into the box. There are kits available at home centers, intended for small insulating jobs, that have tanks of foam and nozzle that mixes components as you inject. Allow provision for excess foam to squeeze out of the box and run over its edges, so you will have a clean line between the two halves of the eventual helicopter. Best to add shapes to the mold halves that will create fiberglass rim or tabs on your final fiberglass copter halves to make it easy to attach the two halves together at their seam. Once the injected foam sets, remove the male helo plug and use same method to make mold for the other half. U.S.

xtricity (author)2007-09-20

There's some good information here. Thanks for your work in putting it up. Can someone give me more information about using the plastic sheets as a way to release things from the mold or clamp? I've fiberglassed using epoxy before, but I wasn't aware that plastic sheeting could be used to clamp the fiberglass cloth onto the surface while the epoxy dried. Is this like a plastic drop cloth? Do I need to prep the surface with anything? Can I take the plastic off the epoxy without sanding it off?

finewoodenboats (author)xtricity2008-03-20

Some great tips here. I use wax paper.

unclesam (author)xtricity2007-09-21

Xtricity, the plastic film is just thin cheapest plastic dropcloth, one layer, from Sprawlmart or home center. No surface prep or mold release is necessary. Once epoxy has completely cured, plastic sheet will peel off. Wherever there has been a fold or crease in the plastic sheet that epoxy wicks into, plastic sheet may tear at that place, and trapped plastic will need to be sanded off. U.S.

psychsurf (author)2007-06-15

Nice tips! I've done a lot of work with glass/epoxy on surfboards, so I need to add a safety comment here. Even though acetone works well to clean up epoxy, it is the last thing you want to use from a safety standpoint. Acetone allows epoxy to penetrate the skin and into the blood. Most cases of epoxy reaction/sensitivity in the surfboard and boat construction industries have been traced back to skin exposure to epoxy-contaminated acetone. Denatured alcohol works very nearly as well, and is infinitely safer. To clean up those yellow spreaders, I've had great luck just leaving it on and then flexing them once the epoxy is totally cured. In my experience it comes off in big sheets, easy cleanup.

unclesam (author)psychsurf2007-06-16

Waterless hand cleaner, used following the directions in the linked article, prevents epoxy on tools from ever setting up. Cleanup can be done at liesure and requires only hot water, soap and a brush. If hand cleaner gets on your skin, no harm is done. I worried that acetone would attack the rubber and plastic parts on some of my tools, and I prefer to use it only sparingly for other jobs.

quigs999 (author)unclesam2007-06-20

Vinegar does the same and is even cheaper. I keep an old pot full of vinegar near my fiberglassing table, and drop my squeegees and mixing sticks into it. After I have several in there I clean them all at once. If you clean your gloves with a bit of vinegar you can reuse them without having them get all gunked up with dried epoxy.

flywoodkb (author)2007-06-15

Good info, thanks!

I usually mix my epoxy on a cheap kitchen scale rather than by volume. I use cottage cheese containers. Epoxy pops out of those pretty easily after it cures.

I'm looking forward to the FOAMBOAT instructable too! I think I want to use that method to make the ama for my already-in-progress kiteboat.

unclesam (author)flywoodkb2007-06-16

FOAMBOAT Instructable has been published. Enter "unclesam" in search box to easily find it.

Leon Close (author)2007-06-13

Fast looking hulls. What sort of craft will these become?

unclesam (author)Leon Close2007-06-14

These hulls were designed to be used as a two-hull stable platform for experiments with unconventional human-powered propulsion mechanisms. The solid molds were made using a method I developed and call FOAMBOAT, which allows you to go from sketch to hull quickly and easily. Stay tuned for Instructable of the same name, within a few days, plus some other related ones. It takes me a while to convert these things to Instructables format. Just for you, a sneak peek, click on and and Double-click to enlarge each image, download to read the fine print.

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