My Boat Repair / Fiberglassing Guide

Hey All,

I wrote this up for my boat club as I was teaching some people how to repair their boats.

I figured it may be of use to others as well. Please see the posts below, as the write up is somewhat lengthy



Composite Repair supplies list

  1. Clothes that you don’t care about or Apron
  2. Disposable gloves - not thin / cheap ones, something thicker.
  3. Acetone, Denatured Alcohol, or Lacquer thinner
  4. Painting Rags
  5. Dust Masks for sanding
  6. 80 Grit sandpaper (rough sanding / initial smoothing)
  7. 120 Grit sandpaper (intermediate smoothing)
  8. 150 Grit sandpaper (intermediate smoothing)
  9. 180 Grit sandpaper (semi-finish sanding)
  10. 220 Grit sandpaper (semi-finish sanding)
  11. (possibly 320, 500, 1000, and or 1500 grit sand paper as well if you want a semi-polished look)
  12. Craft paper / cardboard (template/pattern for cloth cutting)
  13. Masking Tape (Intertape brand is by far the best. 3M, bluehawk, and store brands are Junk)
  14. Masking Paper (6” or more. 8” or 12” is recommended)
  15. Resin Part A (West Systems 105 Part A)
  16. Resin Part B (West Systems 205 Fast Hardener for 55-70° weather)
  17. Resin Part B (West Systems 206 Slow Hardener for 70+° weather)
  18. Digital Scale, 1 gram or .1 gram precision
  19. Mixing Cups (Link) (Also available at most grocery stores, or anything works)
  20. Mixing Sticks (anything – popsicle sticks, paint sticks, ect)
  21. Disposable ‘chip brushes’ (Link) (not foam brushes)
  22. Carbon fiber cloth (the strongest and lightest fabric)
  23. (or) Fiberglass “S Glass” cloth (if you want your patch to be clear-translucent)
  24. Peel Ply (to smooth the patch and squeegee resin out after applying resin)
  25. Flexible squeegees (to remove air bubbles and excess resin)
  26. Resin Thickener (for filling creases or holes)
  27. Sharp Scissors (not the crappy pair from your house. A dedicated, sharp pair)
  28. Long bladed, light duty utility knife
  29. Short bladed, heavy duty utility knife
  30. Extra knife blades (for rough trimming semi-cured resin and other things)
  31. Putty knife (1.5” or 2” works best)
  32. Spray Paint or Spar Varnish (something to block UV and match boat color when applicable)


– Last Updated: May-13-16 1:33 PM EST –

Note: Important information to consider may be in the few steps after the one you’re reading or acting on. Read ahead 2-4 steps so you know what the next step is and if you need to consider anything in addition to the step at hand. Please read all the way through the instructions prior to starting

1. Put on junk clothes or an apron, rubber gloves, wet a rag with solvent very well

2. Rough clean the repair area

3. Get a new rag, wet, and finish clean area again
Failure to clean the repair area well may result in the resin being repelled resulting in “fisheye” or waterfalls

4. Sand the repair area with 80 grit – Extend sanding 2” beyond the actual damage (in most cases)
Important – remove all sticky residue by scraping, sanding, then solvent wash, or epoxy will not adhere well
Important – in your mind, define the boundaries of the repair and take care to not sand outside this area. It will look sloppy if you sand haphazardly. You can quickly tape off the area for the cleanest look.

5. After sanding is complete, thoroughly remove all the dust with a dry brush or a wet rag

6. Remove the “temporary sanding tape” if you applied it and tape off the area around the repair with masking tape, following your defined sanding area

7. Apply a perimeter of masking paper around the tape to catch drips and runs, and allow a surface to squeegee the excess resin onto. Tape edges down so they can’t be blown up by the wind

8. Take your craft paper (or a paper grocery bag works ok too) and cut out the pattern of each patch. If multiple layers of cloth are needed (like for repairing a puncture) start by cutting out the largest patch based off your taped-off area (¾ “ smaller than the tape, remember) , then make 1 or 2 smaller templates based off the largest one you just made.

9. Take your templates and Cut out the appropriate shapes (patches) from the Carbon or Fiberglass cloth
a. If multiple layers are needed, make a “Bulls eye” of cloth layers where the smallest layer is almost the same size as the damage, then make a larger layer that extends beyond the damage by 2” if practical (in tight areas, a lot of extra cloth overlap is not always practical)
b. If 3 layers are used, make the middle layer half way in between the size of the smallest and largest patch.
c. Cut the cloth about ¾” smaller than the taped off area on all sides. If the cloth extends over the tape, it is hard to remove and tape obviously creates a bad bond, and the patch will peel off over time. Make sure no cloth extends over the tape!
d. Note: the cloth will grow and stretch when you apply resin so make sure you allow a ¾” to 1” buffer on all sides between the cloth and tape

10. Once the cloth is cut out, carefully set it aside in an organized staging area. (Once the resin is mixed, you don’t want to waste time figuring out which piece goes where or replace damaged pieces because the clock will be ticking)

11. Cut out Peel Ply in shapes that extend beyond the patch about 3” on all sides and place with the matching pieces of carbon cloth.
Note: Prep is 70% of the work! A half-assed prep job will lead to a half assed repair!


– Last Updated: May-13-16 1:35 PM EST –


1. Gather all the necessary supplies and organize them so that everything is staged and easy to access. You’ll need: Pre-cut cloth, pre-cut Peel Ply, Resin part A, Resin Part B, scale, stir sticks, masking tape, extra gloves, chip brushes, garbage can

2. Decide if you’re going to use 205 fast hardener or 206 slow hardener. If the temperature is below 70°, you should likely use 205. If it is going to be below 60°, you must use 205. If the temperature is in the 70° or higher, 206 will give you more working time. If you’re a novice, I recommend 206 because it allows more working time before it gels. Also consider the following factors:
a. Resin cures faster with higher temperatures
b. The working time of resin varies a lot depending on how much resin is mixed at once, the shape of the container, the temperature outside, exposure to sunlight, the temperature of the surface to be patched, and how quickly the resin is spread out on to the surface or fabric. (more surface area = more heat radiates into the air = less retained heat from the exothermic reaction of the Epoxy)
c. In general, you can expect 5 minutes working time with 205, and about 10-12 minutes with 206, less in hot weather, more in cooler weather.
d. The curing action of resin is Exothermic, meaning it creates its own heat. This is important because if you mix a lot of resin together (even 8oz in a cup), it can get very hot and gel very quickly, making the resin unusable. Remember that large batches of resin can cure very quickly!
e. Once resin “Kicks” or begins to turn from a liquid to a gel, it becomes unusable to saturate cloth. Be aware of when your resin begins to gel and don’t try to saturate cloth once you notice it beginning to gel. It will not saturate cloth once it begins to Kick. Once you notice it, you have very little working time left! In some cases, just a handful of seconds if its warm out.
f. Partially ‘Kicked’ resin can be used to back fill holes if you do not have resin thickener
g. In general, use 206 hardener if you are new to composite repair and the temp is above 65 for at least 4-6 hours.

Note: You can lay down multiple layers at the same time, as long as the resin has not cured and ‘blushed’. Blushing is a chemical process whereby Amines come out of solution and create a cloudy appearance on the surface of the cured resin. ‘Amine Blushing’ happens near the end of curing (when it is in the solid state) and inhibits the ability of subsequent layers of resin to adhere to the first layer. To ensure proper bonding between layers, lay up all layers at one time before blushing occurs, or, wait for the first layer to fully cure, wash with acetone, sand with 80 grit, and layup the next layer. You can apply layers back-to-back as long as you do it in the same Glassing session (generally you’re ok to apply layers back to back within an hour or 2 of eachother)

3. Put on clothes that you do not care about. Resin ruins any clothing it touches and will not wash out. Ever.

4. Prepare a chip brush: cheap disposable brushes often shed bristles during use. Pull on the bristles with moderate force and shake the brush out to remove the loose bristles. (When the bristles come out during the repair, they’re annoying to remove when soaked in resin).

5. Put on disposable gloves. Stage a 2nd pair in case you rip one. Resin covered fingers are no fun.

6. Decide if you need to back-fill any deep grooves, holes, creases ect.
a. Creases need to be back filled if, when the cloth is pulled tight, there would be a pocket of air beneath the fabric. This is most often needed on convex surfaces (like the bottom of a boat) when they take an impact. Often, the impact will create a divot in the hull. If the cloth is just stretched over the recess without back fill and an air pocket is created, that will create a weak spot that is likely to puncture again.
b. To back fill a divot, mix a small amount of resin (probably Less than 18 grams total) as described below, and stir in thickener until you have the consistency of mayonnaise.
c. Get a popsicle stick, paint stir stick, or something to smoosh the filler into the divot
d. Use a flexible squeegee to smooth and flatten the thickened resin. It should be even with the undamaged surfaces adjacent to the divot
e. Wipe off any excess thickened resin from around the fill area with the squeegee or putty knife

7. If you have patches on multiple surfaces, decide which surface you will repair first.
a. Convex surfaces (like the bottom of the boat) is easiest to start on, because you can tape peel ply tight against the boat to keep the patch in place when you flip the boat over. If you have multiple surfaces to patch that are concave, you should position the boat so that gravity will pull the patch into the recess and wait for the resin to gel before moving the boat. This will prevent the patch from pulling away from the repair area.

Mixing Resin

8. Turn on the digital scale, change it to grams, zero the scale with the mixing cup on it.

9. Add 5 parts resin to 1 part hardener by weight (its better to mix multiple small batches than 1 big batch because of the exothermic reaction – large batches will cure very quickly because of the heat produced by large batches of resin)
a. Examples:
i. 5 grams resin, 1 gram hardener = 6 grams total
ii. 25 grams resin, 5 grams hardener = 30 grams total
iii. 50 grams resin, 10 grams hardener = 60 grams total
b. As soon as you add the hardener, you’re on the clock! You only have 5-14 minutes to use the resin so move efficiently, but not hastily.

10. As soon as the hardener is added, vigorously stir the mixture for about 30 seconds, but try not to add too many air bubbles (some bubbles are fine, but you don’t want it to look white and foamy) be sure to get all around the bottom and sides, because unmixed resin will never cure!

11. Brush a light layer of resin onto the surface

12. Lay the first layer of cloth onto the surface (remember, start with the smallest patch first for multi-layer patches)
a. Try not to move it around too much as it will tend to fray around the edges and distort the shape

13. Brush on enough resin to saturate the cloth. Fiberglass will become transparent when wet out. Carbon looks the same wet or dry, you just have to go by feel or you can look for the wet sheen on the surface as an indicator that it is sufficiently wet
a. It helps wet out the cloth if you ‘push’ the resin into the cloth as opposed to brushing it. Use brushing motions to spread the resin (not around the edges though) and pushing motions to ‘push the resin in’ and saturate the cloth.
b. Use caution around the edges of the cloth, as the brush will pull strands of the fiber out of the cloth weave, leaving you with a distorted shape or stringy hairs coming off. Only use a pushing or dabbing motion around the edges. Try to maintain the original shape as much as possible and avoid “hairs” coming off the cloth towards the edges.
14. If your resin Kicked (gelled) mix a new batch

15. If you can still use your first batch of resin, lay down the 2nd layer of cloth (if needed)

16. Wet out the 2nd layer with resin in the same way as above (if needed)

17. If your resin kicked and you have a 3rd layer, mix new resin (if needed)

18. Lay down the 3rd layer (if needed)

19. Wet out the 3rd layer

20. Quickly manicure the patch; Make sure it is pressed securely into concave corners, as it can pull up if you move the patch around during brushing. Poke at air bubbles between layers with something sharp (skewer or tip of the utility knife) and see if you can work them out. Don’t take too much time though, because the peel ply needs to be applied while the resin is still liquid or in its early gel state.

21. Lay the Peel Ply over the patch, extending beyond it on all sides.

22. Take a flexible squeegee and carefully wipe the excess resin from the middle towards the masking paper on all sides of the patch. Be sure not to pull or move the fabric with the squeegee. Hold the peel ply in place with your other hand to provide an anchor for it.
a. The goal here is to remove as much resin and air bubbles as possible while still having wet cloth. Resin adds no strength, only weight, and excess resin actually makes the patch weaker. Wipe the resin outside the taped off area similar to how you wash your car windshield with a squeegee.

23. Once the resin is squeegee’d out, you can tape the peel ply in place to prevent the wind from blowing it, or, if you’re doing repairs on both sides of the boat, it will hold the patch in place until it cures.

24. Repeat the Mix resin/apply resin/Lay cloth/Wet out/Peel Ply/squeegee/tape-in-place procedure for the remaining patches
25. Clean up!

a. Watch for runs! If you can find them and wipe them off while they’re liquid, it saves a lot of work later. It’s ideal if you can look back at the patch every 10-15 minutes until the resin stops flowing.
b. If you need to get resin off your hands, Acetone or Lacquer Thinner is about all that will work, and even then it doesn’t work great. It’s much better not to get it on you in the first place!


– Last Updated: May-13-16 1:36 PM EST –

Removing the peel ply and shaping the resin while it’s still semi-soft

1. Wait until the resin is just barely tacky to the touch, probably between 2 and 6 hours after mixing depending on the temperature and which Hardener was used

2. Slowly peel up 1 corner of the Peel Ply. The resin on the boat should not move/deform, should not pull up, and should not lift any fabric with it. If the Peel Ply lifts any resin or cloth from the boat, the resin is not cured enough yet. Wait another 30-60 minutes and check again.
a. When I say ‘lift from the boat’ I mean the resin will create small hair like strings similar to melted cheese strings on a hot pizza, or pulling gum off your shoe. Or, the peel ply may pull a string of cloth up out of the resin. This means its not cured enough. Wait!

3. Once you can peel up a corner of peel ply without lifting any fabric or resin, you can remove the entire sheet of peel ply. The surface should be relatively smooth, probably with a few wrinkles in the resin if the peel ply bunched up.

4. Pull the tape – carefully!
a. Pull the tape slowly and watch for strands of fabric that were caught outside the patch area. If a strand starts to pull up, take a sharp knife and cut it as close to the boat surface and edge of the patch as possible
b. If any fabric got pushed over the tape, you must take a very sharp knife laying down almost parallel to the surface, and cut towards the patch until you get through the tape. From there, angle the knife upwards and cut out of the patch. Obviously, you don’t want tape to be the primary bond of your patch! That would create a weak spot and the patch would start to peel from there almost immediately!

5. If you catch the resin at the perfect time, you can shave off any high spots with a sharp knife. Its easier to do it now than when it cures! (“The perfect time” is when the resin is hard enough that the fabric cant move and the resin is solid, but isn’t fully cured. A good test is to try to dent the resin with the tip of your thumb nail. You should be able to somewhat easily dent the resin, but it shouldn’t be overly sticky or come off on your finger)

6. If the resin hit a high spot in the tape and has a blunt/sharp edge, now is the time to cut it down at a shallow angle. It will make smoothing with sandpaper much easier tomorrow! Also, it is nearly impossible to sand a smooth blend from a blunt transition line. If you can even get a little taper to the edge, it makes blending easier and better looking in the end.

7. Let the resin cure fully (Usually about a day. It can be as little as a few hours in hot weather, or up to a couple days in cool weather)

Final smoothing and sanding

1. For Flat or Convex surfaces get a rigid (stiff backed) sanding block and load it with 100-120 grit sand paper

2. Try to knock down the high spots in the patch with the coarse sand paper

3. Once the high spots are flattened smooth the patch with 150, 180 or 220 grit sandpaper
a. If you’re painting the patch, you can probably stop at 150 or 180 grit depending on how nice you want it to look
b. If you’re going to clear coat the patch, you probably want to sand it to at least 220 grit or possibly 320 if you want it to look smooth

4. It is recommended that you paint the patch with either an opaque paint or at minimum a clear Spar Varnish to provide UV protection. Over time, Ultraviolet light from the sun will degrade the epoxy and cloth. After about 600 hours of sun exposure, the resin becomes about 30% weaker, so it’s a good idea to clear coat or paint the patch.

Glassing Part 2

– Last Updated: May-13-16 1:31 PM EST –

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