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

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

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

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

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

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

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

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

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.



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    37 Discussions


    3 years ago

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

    1 reply

    3 years ago

    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.

    1 reply

    5 years ago on Introduction

    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.


    11 years ago on Introduction

    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 ;)

    2 replies

    Reply 9 years ago on Introduction

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


    Reply 6 years ago on Introduction

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

    Ellen the Generous

    6 years ago on Step 5

    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?

    1 reply

    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


    10 years ago on Step 8

    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?

    3 replies

    Reply 10 years ago on Step 8

    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.


    Reply 10 years ago on Step 8

    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.


    Reply 6 years ago on Step 8

    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!


    8 years ago on Step 4

    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.


    8 years ago on Introduction

    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.

    1 reply

    9 years ago on Introduction

    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.