I was asked to write a basic how-to article on how to fiberglass an existing hull, and while I’m not an expert, I’ve done this enough times over the last 30 years to handle it with reasonable confidence.

I do this to reinforce weak hulls and to ensure watertightness. Depending on your boat, you may or may not want to fiberglass – but I like it. The pond I used to sail at was notorious for people with marginal piloting skills. I really didn’t want any of my boats caved in by some yahoo.

And, before I start – I wanted to mention fiberglass splinters. If you use woven glass cloth, not mat, and if you handle it with reasonable care, you’ll won’t have any. And, in any case, if you handle cloth remnants wearing gloves, there won’t be any issues. Use of mat, and hacking at heavy grade glass cloth can make splinters – but there’s simply no need for that here.

For a light 1m model, one layer of 50g ‘glass is enough. For a heavy tug or a larger model, I will probably use 2 – and I might even go to a heavier cloth if I feel the need. One can simply add layers until one is happy.

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Glass cloth - There are 4 basic kinds of cloth sold in my area: 1) mat 2) “E” glass; 3) “S” glass; and 4) mat and woven together.

The best for our purposes is “E” glass – it’s the ordinary, less expensive kind anyway.

“S” glass is stiffer and stronger than “E” glass. We don’t need the strength, but we want that flexibility when working around the tight curves of a model boat hull.

Mat is stiff, gives off fibers, is weaker and heavier, and is harder to work with. It exists to provide a surface on the outside of a layup that does not have a visible weave – but we’re not only working with MUCH finer cloth than the bug boat folks, but we’re going to fill the weave with resin anyway. The combined and woven has the same problem.

Glass cloth is sold by the yard in different weights. I’ve used everything from 18g (per sq. m.) to 150g. I prefer 50g or 60g.

18-20g cloth is extremely flimsy, but super-flexible. It has to be handled VERY gently and as little as possible, as the weave just falls apart. It’s a good choice where the hull curves are extreme and you don’t want to cut “darts” in the cloth. One layer of this IS enough on a small, light boat like my Midwest 23” lobster boat. It’s been over 30 years since I built it with 1 layer of 18g cloth over thing balsa sheet and it’s doing just fine.

50-60g cloth is a nice compromise. It’s light enough to go around reasonably tight curves, and heavy enough to be a little easier to work with. It’s what I normally default to – though it’s lighter than many folk use.

If the curves aren’t particularly tight, or you need more strength cloth up to 150g is just fine. It takes a little more work to fully fill the weave, but it’s still a very reasonable weight. A lot of people go straight to this weight because the cloth itself is a little stronger and easier to work with.



Resin –

The two types commonly used by modelers are polyester and epoxy.

Epoxy has significant advantages … but it’s a lot more expensive. I still recommend it.

Most important – polyester gives off toxic fumes that cause brain damage if inhaled in quantity over time.

Also, unlike polyester, the epoxy I use can be had with different hardeners to speed up or slow down the working time and curing time.

Lastly, while I have not experienced this, it’s my understanding that polyester resin shouldn’t be used over epoxy. But, epoxy can be used over polyester. To me, it makes sense to use the more compatible material.

If you DO want to use polyester resin, I strongly recommend using it outside, or at least in a space that is well ventilated and that you can stay out of while the resin is curing. You should always use epoxy resin in a well ventilated area as well, but the level of toxicity from fumes is substantially lower, as is the odor. Epoxy has some odor, but it is not particularly strong, while polyester resin just has an unpleasant penetrating smell – which a good thing as you know that you have to leave the room!


Cleaning and precautions –

Resin cleans up with isopropyl alcohol – that takes it off nice and easy if you spill any or get any on you. Soap and water won’t work.

This is a PPE zone – personal protective equipment is important here. You don’t want to inhale resin/glass dust. Microballoon inhalation is dangerous. You don’t want to inhale the toxic fumes that polyester resin gives off. And, skin exposure to either polyester or epoxy can trigger allergies or skin sensitivity. So, try not to get it on you.


Prep –

Resin is tough and slow to sand, and the glass in the cloth will dull your sandpaper quickly. So, get the hull as right as possible before ‘glassing. If you’re planning on a clear ‘glass job with visible wood, I’d sand to about 220 grit. You can probably get away with 180. But, 120 or coarser will leave lines on hardwer woods. If you’re going to paint the boat, then 80 grit is fine.

Clean and de-grease the hull as necessary.


Layout –

You can cut glass cloth with sharp scissors. They won’t last long. So, plan on re-sharpening or replacing regularly if you do a lot of ‘glass work. I keep a cheap pair on hand for this.

You have to decide how many pieces of glass to use based on the shape of your hull.

For hulls made from flat sheets, as a general rule, one piece of glass per flat surface unless there are a lot of flats with fairly shallow angles between them. You need a separate piece of glass every time you hit a sharp angle.

As a rule, glass cloth is good at following inside curves or moderate outside curves. If you don’t mind a little fillet, you can get ‘glass to go tightly into a right angle bend – say where a keel meets a hull. With 50g ‘glass cloth, that’s about a 3-4mm fillet. But, it won’t stay flat on a right angle outside bend – so you don’t want to lay a single piece of glass across the width of the bottom of a boat with a skeg. The glass just won’t lay flat on the bottom of the skeg.

So, on a hull with a skeg, you’ll probably going to use 3 pieces (4 if the boat has a transom). There will be one on each side of the hull and there will be a little strip along the bottom of the keel – if the keel sticks out. If there is no external keel, then no center piece

Which order you apply the pieces is not critical.

Pre-cut each piece before application, and until you get the feel for it, do leave yourself a few inches to spare all around. Too much extra material will get in the way. But you might have to stretch the material in an unexpected direction to get round a curve and you don’t want to come up short – if you do, you’ll have to lay up a patch and sand it all flat to match. No real harm done, but it takes time.

Note that there is a limit to how much you can get glass cloth to squeeze together. If a hull gets smaller toward an edge – say on a hull with a lot of tumblehome going around a rounded stern, there may be a spot on a hull where there is simply going to be a crease. I don’t attempt to deal with this when I’m cutting out raw pieces of cloth. My fix to that is, when I’m applying resin and I see that, I’ll get a fresh x-acto blade and slit the crease. There will be a bit of an overlap, but it’ll sand off nicely. If you are going for a clear finish, make sure not to make a mark on the underlying wood.


Mixing the resin –

The most important thing to know about resin is that it’s exothermic. It will heat up. And the more it heats, the faster it kicks off. If you leave more than about 100gr in a cup, it will eventually heat up enough to deform a plastic cup. It may bubble, and in extreme cases, if it’s thick enough, it can catch fire. So, don’t mix up more than you need, and if you need quite a bit – pour it into a wider, shallower, container if possible. And, if it is heating up, either epoxy or polyester is going to give off seriously toxic fumes. If you have plenty left over – let it harden outside!

Polyester is fairly forgiving about getting the right amount of hardener, but epoxy can be finicky. And, not all epoxies mix at a 1:1 ratio. 2:1 and 5:1 are common. If you’re dispensing small amounts, it’s best to dispense into a cup on a scale, first one and then the other into the same cup. I suspect it’s supposed to be done by volume, but I’ve measured into 2 cups by volume, poured one cup into another, and had the ratio be off enough that the resin did not cure. I haven’t had the same problem dispensing by weight, but that’s only been with West System products – so your mileage may vary.

West System products can be had with volumetric dispensing pumps. These are fantastic. One pump of resin goes with one pump of hardener and you have about 25g of product. But, getting less is tricky – you can do partial pushes of the pump over a cup on a scale.

The other downside is that not only are the pumps expensive, they work with the gallon size of the resin – and a set of that quantity of resin, hardener and pumps is well over $200 in the US. It DOES have a good shelf life – I’ve gone 6-7 years without issue, though 10 was too much. The good news is that if you regularly use them, the pumps will last for many gallons of resin and hardener. So, if you’re eventually going to use it all up, just go ahead and invest.

The resins are, of course, available in smaller quantities, and you just don’t get the pumps.

Mix the resin well. As a rule, if you stir until it appears fully mixed and then stir for another minute, it’ll be just fine. I find that if I don’t stir that extra minute, some areas of resin may stay sticky for an extra day – though it’s always cured just fine for me eventually.

Resin cleans easily with Isopropyl alcohol.

You can also thin resin with this material – but don’t do it. Every 10% of alcohol or other thinner you add will take away approximately 15% of the strength of the finished resin after it cures. If you need the resin to flow better, heat it with a hair dryer and be prepared for it to kick off quite quickly.

I plan on about 3oz of resin to do one side of a 1m boat hull. Experience will tell you if you need to adjust that amount.

That's all I can put in 1 article. I'll put the rest in a part 2.