Fibergass Repair Tips

Article By Shorty
Misc Fiberglass Repair Tips
I restore sailboats a bit differently than a typical fiberglass shop would, the techniques I use are rather simple and rough looking. I prefer funcion over beauty. Here are some tips that I might not have explained on other web pages.

Basic Fiberglass Repair Steps
Fiberglass repairs are a bit like doing paper machet. What you do is sand down the surrounding area to roughen up the repair area. Then you put fiberglass cloth soaked in epoxy on the area and wait for it to cure. If it is a deep hole, you put down enough glass to fill it back up. Then after the epoxy is cured, you sand down the repair to improve it's cosmetic apearance, and then apply your finish coat over top of the repair -- like paint or gelcoat. That is it, pretty simple. There are many techniques you can do while repairing, like multiple layers of glass in multiple sessions, special sanding techniques, special fillers to thicken the epoxy to make the sanding easier etc - but all of those techniques are all aimed at cosmetics to make the repair invisible after the job is done. Structurally sound repairs are very easy to make. Repairs that cosmetically completely dissapear are rather difficult to achieve.

When I go into a marine store to get some "stuff" for repairs, I don't know what to get. The local store has "West Marine" stuff and 3M stuff and other brands. Which are the good ones? Which should I avoid?
There are a lot of products available, and many of them have confusing labels. I use regular epoxy - specifically I get the stuff which you mix at a 2 to 1 ratio, and is a "no blush" formula. Many older epoxies create a slimey surface 24 hours after they cure called a "blush". If you put a layer of epoxy down and wait more than 24 hours, you need to clean this blush off before putting on the next layer. If you purchase "no blush" epoxy, you don't have to worry about it.

When you do put down multiple layers of epoxy (and fiberglass), you should try to get additional layers down within 24 hours of the previous layer, even though you are using no blush epoxy. The reason is because there is a 24 hour window where additional layers will chemically intermingle & bond to the previous layer. If you wait longer, you will only have a mechanical bond (which is still really strong), but you should sand the previous layer if waiting this long.

Does epoxy have a shelf life?
I am sure it does have a manufacturer rated shelf life, so ask when you are buying, but I know from personal eperience that epoxy keeps for a long time. When I was a kid, I used to build models and my dad would buy small kits of epoxy. There was one kit that I used a little bit of, then it sat in his basement for over 30 years. I discovered this kit a couple of years ago and used it, the epoxy set up just fine and is still holding on one of my boats.

Don't use Polyester Resin to make repairs
Your boat is probably made from fiberglass cloth that is soaked in polyester resin, which is a great material to mold boats from. It sticks very well to itself during the first 24 hour curing window, is UV resistant, and doesn't rot. The bummer thing is that polyester resin isn't very good at secondary bonding - meaning it won't stick very well to an existing surface. I have made repairs to boats with fiberglass and polyester resin, and after a few years of flexing in the water, the patches start to pop off. Epoxy sticks very well to sanded polyester resin and makes a permanent repair.

How to tell if product is polyester resin, or epoxy? With the many product & brand names, it can be tough figuring out what is epoxy based, and what is polyester resin based. There is one easy way to tell - that is by the mixture ratios. Epoxy needs to have large quantities of 2 parts mixed, such as 2 parts of one fluid with 1 part of another fluid. Polyester resin based products have a large amount of one material, with jut a few drops of another. Bondo is a polyester resin based product, it has a bunch of fiberglass fibers mixed in so is more like a paste.

How do I know how much epoxy to mix up?
The method I used for mixing epoxy is to use a postal scale. I put a disposable plastic cup on the scale and then put in one part. Read the weight, then put in enough of the other part according to the weight I need. So if I put in 2 ounces of the part 1, then I add part 2 epoxy till the scale reads 6 ounces. Then I know I have the proper ratio -- 2 of part 1, and 4 of part 2. Then mix really well with a popsicle stick. Like make atleast 50 (fifty) good turns with the stick and get all the corners of the cup mixed up well.

Wanna see something neat? After the epoxy has set up hard, try flexing the cup and popping out the epoxy that cured in the cup. As you mixed it up, the popsicle stick scratched the bottom of the cup, and those scratch marks will show up in the cured epoxy, plus any other neat stuff that was molded into the bottom of the cup. Congratulations you have just molded your first part, that is basically the way boats are made but on a larger scale.

Don't use Silicone or other gummy products
I have seen SO MANY repairs to boats, where there was a little crack and the owner just took some silicone and spread it over the surface of the crack to repair it. That silicone does not stick to anything but itself, and so the water will still leak thru that crack. Then to do a proper repair, the silicone needs to be dug / ground / sanded out of the crack later to put epoxy on for the repair. There are other gummy products that people use for repairing fiberglass, and they are all annoying. Epoxy is so easy to work with, just use it.

Dry your fiberglass before making repairs
(say what? Fiberglass boats soak up water?)
Strange as it sounds, fiberglass and polyester resin (or epoxy) DOES absorb water. One neat experiment you can do is to take a square piece of plastic wrap and tape it onto the side of your hull after it has been in the water. You will be able to see the fiberglass sweat out moisture. Take the plastic wrap off, wipe it dry and re-apply the plastic square to see if it sweats some more. When it stops sweating, your boat is probably dry. It is a good idea to dry your boat before making repairs, or atleast after you grind down the area for repair, let a fan blow on it for 24 hours before putting the new epoxy & glass on.

What fabric weight should I get? For small cracks & holes, I really like fiberglass cloth in 4oz to 6oz. 2oz is super light, and 8oz tends to be tougher to bend around curves. If the repair is going to need a thick build up, you can use multiple layers of cloth stacked up on each other. I also really like the fiberglass tapes - they don't have adhesive to them, they basically a narrow row of cloth that has a finished edge on the sides. My favorite is 4" wide tape at 6oz. Fiberglass also comes in a form called "mat", which is random fibers pressed into a flat surface, this is for boat molding and finish work, and there is also a really thick fabric called "roving" and is for molding boats also. For simple repairs, you won't need either of these.

How do you put on layers of fiberglass?
Most of the time I make the fiberglass cloth wet, then apply it to the repair area. Here are the steps:
- prepare the repair area by sanding it down
- lay dry fiberglass on the areas that need repair, and cut pieces to fit
- if I need multiple layers of cloth, I keep those pieces in their proper order
- mix up my epoxy in a plastic cup
- lay the fiberglass cloth(s) on a piece of cardboard, and use a paint brush to make the cloth wet with epoxy
- use the paint brush to make the repair area wet with epoxy
- pickup the wet glass and lay it on the repair area, and smooth with the paint brush and squeegee, make sure to get all the air bubbles
- squeegee out excess epoxy too

I always wear protective gear like disposable gloves, face mask, safety glasses, apron, and have a fan blowing fresh air onto my face. I make squeegees by cutting squares from the sides of flat sided plastic bottles, or use squares of cardboard. To keep the repair area from growing, I mask off the surrounding area with masking tape and newspaper. If the epoxy is really thick, mix in a little acetone (like up to 5% of total volume) to thin it out, will help the epoxy penetrate the cloth better. Sometimes when using long strips of fiberglass tape, I'll dunk the piece of tape into a bucket of epoxy, then squeeze it as I pull it thru one hand to get the excess epoxy out of the cloth.

How do you repair small cracks that aren't big enough for a piece of fiberglass cloth?
For small cracks, I sand down the crack and then use just epoxy thickened with a filler. Start of with a little bit of epoxy and mix in your filler till it looks like peanut butter. The dust that collects in a belt sander catch bag is great, bread flour works. Sawdust is a bit rough, if you run it thru a screen, you can get some good stuff from it. You can also buy real fillers for various purposes, but I tend to use stuff around the shop.

How do I repair a hole, that I can't get to the back side of?
The problem is that if you lay a bunch of fiberglass on the hole, the weight of the glass will sag into the hole while it is curing. There are a couple of ways to bridge the hole.
Method 1 - quick and easy, but looks ugly
If you don't care how it looks, and want to get it done quickly, just sand down, then bridge the hole with a single layer of fiberglass cloth. After that has cured, put on additional layers of fiberglass to build up the repair to the original thickness for structural strength. It will leave a bubble on the surface of your hull, but who cares what it looks like, I'd rather be sailing than spending all my time making repairs.
Method 2 - more traditional backup piece method
To make a solid surface to lay glass on, first we have to attach something to the back side of the hole. We attach that backup piece by sticking it in the hole, then pulling on it with a piece of string till it cures.
- cut out a piece of cloth that will be big enough to fit over the hole
- wet out that glass and let it cure laying flat on a piece of wax paper - make sure to squeegee all the epoxy out of the glass that you can, we want this to be light and flexible
- drill a hole in the middle of the piece and attach a piece of string
- we the surface of the backup piece with epoxy, and stick it on the hole, then pull on it so it is pressed against the back of the hole -- you can tie the other end of the string to a wall, shelf, your car or whatever to hold it till it is cured
- after the backup plate is cured, clip the string off and go about making the repair
If it is a wierd shaped hole, you can make multiple backup pieces and apply them one after another till the hole is filled.

When you put on a gel coat at the end, do you use a brush, spray, squeegee? Etc.
If you want to use gel coat, you can brush it on and then sand it to get adequate results. Matching the color is rather difficult, even if you have the exact gel goat from the factory because the color changes over time from UV. Professional boat repair shops often will gelcoat the entire hull after a repair to make sure the color is matched. For the small boats, I prefer to just forget the gelcoat, make the repair then paint the entire bottom. The paint I use is oil based high gloss exterior porch and floor paint. It has extra UV inhibitors and hardeners in it and looks great, even if painted on with a brush.

What "self-expanding foam" did you use? I bought a bunch of it years ago and can't remember where I got it, but it is basically the same stuff that comes from the hardware store called "great stuff". Great Stuff comes in an aerosol can and is very convenient to use. Please note that expanding foam is often advertised as closed cell and for floatation, but any foam you use which you either mix or spray, and it expands, is OPEN CELL foam, meaning it WILL absorb water. If you don't belive me, then take some of your foam and submerge it in water for a week or two. Weight it before and after to see how much water it absorbs. The foam is very usefull for many things, just don't rely on it for your floatation.

I do not have a heated space in which to do this repair, so I will do it in an unheated garage. Now that winter is approaching, should I wait until next spring before attempting this?
Epoxy doesn't cure when it is cold, so you will need to heat your repair area. Some people will put a lamp just above their repair, then put a cardboard box over that to keep the heat in. Or have several lamps very close, pointed right at the repair area. Be careful, the cardboard or epoxy could catch on fire. Also you don't want to get it too hot, otherwise the epoxy will bubble and cure too quickly making it softer. The epoxy should come with a data sheet which describes the best curing temperatures.