Steps for fibreglass hull from male plug

Hi all,
Just trying to write down an “Idiot’s Guide” to making a fibreglass hull from a timber plug. Would appreciate if people could please add comments to any of the following steps which are wrong or out of order –

  1. Cut shadows. Allowing for thickness of timber planking.
  2. Mount shadows onto baseboard.
  3. Plank shadows using 3mm balsa strips.
  4. Apply epoxy resin inside hull to strengthen prior to sanding (Shadows still in place but removed from baseboard).
  5. Replace hull on baseboard and start sanding…
  6. Spray paint hull light coat black and sand again using leftover paint to highlight irregularities.
  7. Use combination of filler and sanding to get class ‘A’ finish.
  8. Fibreglass hull plug and let cure.
    To make a hull ;
  9. Apply release wax - 4 to 5 coats for first hull.
  10. Apply fibreglass.
  11. When cured “pop off” finished hull.

Does this sound about right?

I have an order to build 3 x 1m boats so plan to make a plug. Hence my asking for clarification.

Thanx :zbeer:
Larry L.

That sounds perfect; however, it’s the practical, not the theory alone, that makes for fine results.

I recamend that, while you make the plug, also make a practice plug (an old bowl for eg.) to practice your craft until yer satisfied

The key is too never be satisfied, so your mind will continually open to learning, and the quality of your craftmanship will always improve.

As for the materials, maybe shadow someone more experienced, to witness the proceedures in composite construction. That will eliminate a lot of guess-work for the first try. Don’t eliminate too much guess-work though, guessing is good practice for future problem-solving. :slight_smile:

Nigel -
Thanks for that advice on a practice run. I should have mentioned I have had some experience with building and fibreglass. Have scratch built 3 composite hull boats already and refitted 4 more with new masts, booms, rigging, sails. All handmade.
Now I wish to progress and have a plug for one of my boats which is very popular (USOM Mistral).
Often when I sail I’m asked if I build boats. Last time a guy asked me to build him two identical :), for him and his wife. They loved the look of my boat and after sailing it for 20mins were wrapped. She more so than him.
Building two, I figured just as easy to build three - sell them and spend the money on an AC12 I want to build as per Claudio’s plans. And, have the plug for any more orders.
I discovered early that when building I end up with heaps of material left over. Easier to just buy same amount and build 2 boats.

Larry L.

Photos along with sequence of steps may be helpful for those that have not tried before. List materials used & sandpaper grits also.

Right on man. I like the idea of build small, to fund building a bigger one. I want a 30-foot Lajabless…

The idea of stiffening the planks befor sanding the outside is good. Personally, I just give the surface a rough fairing, then I skin it with a thin layer of glass, and appropriate resin (for foam plugs, make sure your resin does not melt it). This gives a solid base / reference, for applying your fairing muds. Tint your muds different coulours, to see what your doing.

When fairing, do your sanding in an x-pattern over the hull, and use good flat sanding blocks (scap carbon plate works well). Your sand paper selection will matter on the densities of your fairing muds. I make mine easy to sand for the first muds, and harder denser muds near the end, that will yeild a good polish-able surface. On some plugs, I paint on a thing coat of graphite-layden epoxy. This makes a good hard surface for watersanding. Only for a plug that needs a hard surface. Some of my plugs, are a simple as covering a foam or wooden shapes, with plastic-type tapes, waxen them up, lay-up right on’em.

Nigel -

thank the lord I found an expert :D!!! Please, please tell me… what is Fairing Mud?
I’m in Australia and a brand name or description of the fairing compound you use would be fantastic(If I can buy it here). None of the brands I have tried have been any good. Especially for a large surface like a boat hull.

I have tried using “Spray Putty”. Expensive and barely fills gaps. And maybe my steps were wrong. I apply filler to bare balsa planking and sand. Hard to work as stuff I tried dries so fast and is a pain to sand smooth. Do you use filler/mud at this stage? And you say apply different muds to fibreglass and then sand to ‘A’ finish. Dry sand, wet sand, finer grades.

And where would I find graphite to mix with the mud? This is all stuff I have never heard of.
And thanks. Sorry if it seems like I’m picking your brain but I’d like to learn how to do it right :zbeer:

Larry L.

Larry -

from my perspective and experience, I highly (and often) reference WEST System products ( Gougeon Brothers - Bay City Michigan - but they have distributors worldwide).

I use their product #410 Microlight Fairing Filler. Mix thin for harder surfaces, mix thick for softer surfaces. (proportion of resin to filler makes the difference - more filler, easier/softer sanding). They also have color tints for resins, and carbon powder to mix in for super smooth surfaces.

While I know both Meade and Jan, I would like to point out that they are the “ONLY” boat builder/boat racers that I know of that use their own product formulations. The list of boats built (big monohulls, small monohulls, Tornado class catamarans, world speed-record contestant, and DN Class iceboats) goes quite long. Should be able to find product at most marine chandleries.

Just my opinion


thanks for that. I have recently found a fibreglass specialist outlet not too far from me that stocks these products.

Just a couple of details. Is mixing the #410 Microlight Fairing Filler with resin the same as the ‘fairing mud’ Nigel refers to? Or is his stuff a proprietary product? Do I have to use West System Products which are very expensive.
Is the #410 Microlight Fairing Filler similar to Microballoons (which I have used).
I figure I just paint this resin mixture on, let dry and sand. :rolleyes: it’s soooooo simple when you stop and think. I kept assuming all this was some kind of “black art” only the initiated were allowed to share. But! Claudio’s thread for his AC33 build - he uses Polyester Filler :confused:. Could I just use good quality wood filler on base balsa, then use fairing compound on fibreglass? Or is this up to builder preference?

I plan to start my plug build tomorrow for my USOM Mistrel. So will hold off on filler choice until I get it clear in my head what is best prior to spending.

Larry L.

Larry, ‘fairing mud’ is just generic slang for fairing filler.

The original checklist you posted is for a lay-up over a male plug - if so, you don’t have to go overboard on the finish of your plug as it will be inside and minor finishing imperfections won’t be visible on the outside of the finished moulding. What is important though is that the shape is accurate and fair - i.e. no hollows or high points, as these will be reproduced in the final moulding.

For my male plugs I tend to use ployester based fillers, they’re cheaper than epoxy and do a perfectly good job. I use a cheap car body filler.

I always use epoxy for the actual mouldings.


Ray responded to your initial question regarding “mud”. FYI - I have even used drywall compound to fair plugs - especially my foam ones since the drywall compound (used for covering seams and nail/screw holes) sands about the same as foam. I try to get my filler to be as close to the substrate as possible for sanding ( hardness wise).

Microballoons are similar and just a bit harder to sand than the #410 Microlight - also balloons are a dark brown which may be an issue. Microlight looks like a very fine, tan colored powder. If you are on a “cheaper is better” project, don’t discount mixing talc (baby powder) as your filler.

The only reason I don’t like polyester resins, is I have one heck of a time controlling their cure time. Having used WEST after all these years, I can pretty much figure out when it will “kick off” and how much time I have left to work the product onto the plug. Yes - it is expensive - but for big boats, the last thing is a glue failure when on the water - :rolleyes: - perhaps not so critical for little boats. It is just I have it, so I use it.

Regardless of what product you use, the object is to have filler that is heavy on the filler, but light on the resin (epoxy or polyester) to make sanding/shaping easier.

Not sure if you have BONDO (or similar - an automotive body filler, polyester based) but a good friend who is a taxidermist (mounts animals for hunter trophies) let me on to using BONDA, but splashing a bit of acetone into the mix to thin it so I can almost spray/brush it on. Sometimes this really thin mixture helps for very light filller applications.

And again a caution - if using foam for a mould, epoxy is necessary. The keytones in polyester resins will attack and “melt” the foam.

Dick :zbeer:

Oh, by the way, now that you know these “super secrets” - someone will probably be by your place ----- you know, - now we have to kill you !

Only kidding of course. :smiley:

Please post other questions of technical concern - I’m sure someone here has had and solved a similar problem.

Ray & Dick :smiley:

thank you both so much for your expert advice ;). This has really cleared up an area that was getting me confused. My previous builds would have been so much easier with this knowledge. My way was creating twice as much work. Used wrong products for filler.

For the plug - the changes / additions to my ‘Idiot’s Guide’ are:-

Step 5A - On bare balsa, after initial sanding use polyester wood filler or other generic wood filler as fairing compound to fill depressions. Sand and repeat as needed. This will become internal face of hull so finish quality not crucial.

Step 12 - Tape hull to plug for curing. Clean with degreaser. Spray with light coat black paint (Claudio’s tracer) and light sand to show irregularities. Smooth on #410 Microlight Fairing Filler (or similar Microballoons etc) and sand to class ‘A’ finish. This is final outer hull.

Now I’m “happy as Larry” and can go shopping.

Larry L.



i have been making plugs and molds know for over 10 years… and i am still learning…
but i will give you a piece of advice a pro gave me…

make a small sailboat ( i ignore this) but i he made sense… make a small boat ( like a wee nip) and start to make the hull
you can use automotive bondo to fair in the big imperfection… but you can also use glazing putty… to fill in the small holes…

the only thing i would do that most people dont. is i finish sanding a plug with 4000 grit… and this is sore… you dont have to go that far… i just do
then i use 10 coats of pva…and polish it… before i start to make the mold…
then i use about 3 -4 coats of pva…

my friend who makes the real models… puts a piece of rope into the plug… so when the mold is done… he can just pull on the rope. and not the plug

also a trick that i found… was once the mold was made… leave it alone for about a week…let it cure… then when ready to part the mold from the plug… use a popcycle stick and pry a little seperation and pour some water in between the plug and the mold
the pva is water soluable. and will help the plug from the mold… make sure it is hot water…
my methods my not work for you… but i have tried it on smaller one… and now build IOM and worked on the marblehead for me

good luck and keep us informed


great tips :D. I do plan to use PVA release and will stock up on paddle-pops! Not too sure about wacking a rope into my plug.
Went shopping this morning and bought sheets of 3mm MDF for shadows plus other bits for new baseboard, shadow mounts etc.
Spent afternoon redrawing profiles for 2 boats. Have PDF’s for about 7 different yachts so redrew profiles for Mistral and Advance MkI. Removed 3mm from profile drawings in Photoshop. Now have to decide if I go for another Mistral or build the Advance :confused:. Probably Mistral. Tomorrow start cutting shapes and mounting to base. Plan to follow methods Kiwi and V8Goose used for their builds. Looks simple and smart. Especially the profiles being held vertical by wood pieces and are still easy to pull out and replace later.

Like in this pic of theirs I borrowed :slight_smile: Sorry guys…

Larry L.

lol great minds think alike… the first boat mold i made… was advance mk 1… i got the shawdows from the amya webpage… and ( to tel you the truth) it did not work outlike i thought… i took some shortcuts…but it did work out…
I will tell you one thing… after you do this hull…
i think what you should . and this wil give you an idea… make a plug for a small thing… maybe a box or bulb. or even a footy…
you can make this from scrap wood left over from your BIG hull
and make a mold of that. with a book of paper beside you
write down your steps…what did you do. how many coats…ect
then makes your mold … from the smaller plug…
this will cut your cost down big time… and if you do screw up… you dont lose alot of money… but if it works… then you go all the info you need for your big hull

now i dont know if anybody has told you… but humidity . has an effect on your mold… what i use… may differ slightly than what dick or claudio use… this is why i suggest making a trial mold… a small boat so that you learn. and if it works… you become a great guy… when you hand the ( ahem ) test hull… off to a kid…
the steps we all use are going to be the same…

now I was thinking should i tell you a secert or let you find it yourself…
but then why not just tell him…
the thicker you make your mold… then better chance you are going to have of it surviving… now most people well make thier molds 1/4 inch think of fibreglass . with wood surport… and that works… no porblem… but if you are near a auto replace place… or a canadian tire store… you can get what is call bondo glass… mixture of fibreglass and bondo, what i did was lay up the mold… and then put feet on it. so that when i work on it … the mold does not move… and cover everything with this bondoglass. when hard… i can almost stand on my molds…
i am part german. part scottish i tend to over kill… my molds work… and you need to just keep in mind… what works for you… and keep us( ahem) experts informed… i would love to see step my step pics… of what you are doing

good luck:zbeer:

Cougar -

more great info from experience :zbeer:.

Now are you and Dick talking about the same thing? Bondo… (Bonda) polyester car body filler. You called it ‘bondo glass’, mixture of filler and glass. I’ll take a punt and say yes.

Just did a Google search for Australia. Bondo seems pretty common from auto shops and hardware stores. Leading product around here for domestic use is Polyfiller (guess similar). Recommended use as wood filler to repair cracks :confused:.

With Bondo - for smoothing onto an already smooth surface, ie hull, what’s the best way to work with the filler. Scrape on with putty knife? Thin and paint on? Spray? Rub in with fingers :p.

Thanks guys
Larry L.

“Normal” BONDO (here in US) is a putty of polyester base. Cougar uses stuff that has short “hairs” of fiberglass mixed in to give extra strength.

I usually put a dab about the size of a small spoon on a plastic dinner plate. Then I splash on a few drops of acetone and mix in until it is of thin consistency - maybe like sour cream for bagels (???) Once I get consistency, I add in the red colored hardener and stir to have an even reddish/pink color. I use an old credit card or polyethelene plastic scrap and quickly spread the filler on as fast and as complete as possible. If you don’t work fast, this small bit of putty/filler will become hard and unworkable in less than 2-3 minutes. Mix only what you can spread quickly. Too much hardener ( drops) and it will “go off” even faster.

Don’t worry if you miss any area, or if you has ridges from the end of the plastic card. You can add a second coat for missed areas, and sand off any high ridges. If you build up really thick, you can use a cheese-grater to quickly fair it down. On flatter surfaces I use a straight edge of broken window glass as a scraper. You will quickly find the right amount of time for hardening, and when the plastic filler can be worked. If you let harden overnight ( or maybe 5+ hours) it will be easier to sand off while if still “green” scraping will work faster. It’s a dusty process so may want to work outside.

While at the auto store, ask them for “SPOT” body filler - usually a small tube about twice the size of toothpaste tube. This is red or green (no matter - just easier to see) and is very fine filler. Good for pinholes or surface imperfections. This stuff really gets hard fast, so I use just a piece of cardboard from a soda case to spread/dab on quickly. This stuff sands even better than the BONDO, and is made to allow feathering in edges of the filler to surrounding surfaces. I think BONDO als makes this stuff too.

This material will ssoak up water/moisture, so be sure to prime and pain before floating the hull. Follow Caludio’s paint plan to find low or high spots.

:graduate: Ha - bet you didn’t realize you would become a somewhat experienced automotive body worker when you decided to try this hobby - did you? :smiley:

Might want to practice a bit on a scrap piece of lumber so you know the set-up and hardening times for the body filler. Wish you lived closer, I’d walk down to your house and talk you through the process. Text and description makes it sound harder than it is. Just dive in and pretend you’ve done it before, is best advice I can give. Soon you will look back on all you have learned and be able to pass it on to local sailing friends.

Hang in there - don’t get discouraged.


PS - once the BONDO gets hard, it will pop off anything of smooth plastic like you spreader cards, mixing plate, etc. Not so much from aluminum sheet, and definitely not from paper or cardboard. While at the auto store, also pick up a small packet of flexible plastic spreaders. Here they cost about $3.00 - and if you have an auto body shop close by, perhaps they will let you watch as they use the body filler - or may even give you a free lesson. Just take your hull plug or finished boat back to show them. They will feel they were responsible by helping you to build your hull.

Larry, what Dick and others have described is just what I had in mind when I mentioned polyester based fillers (i.e. cheap car filler). I don’t see Bondo here in the UK but have equivalents, like Isopon.


Oh, and whilst the short working time of polyester fillers means you have to work quickly there is an upside in that by the time you’ve had a cup of tea it will be ready to start sanding back so you can keep working on your plug without undue delays.

If you use dry sanding there will be a lot of dust and like most things like this it won’t do you much good so think about the appropriate health and safety aspects.


you’re dead right :propeller. Never thought there was so much to learn about car body filler. In Australia this stuff has a very bad reputation in auto repairs. Used to be if you owned an old beat up wreck you would cover all the dents with filler. Give it a quick sand and paint job. The new owner would be non the wiser until much later. Rated as high as bananas and sawdust in the gearbox :bouncy:.
In the past I had been using a filler - Selleys Plasti Bond - Plastic Putty for 1001 filling and bonding jobs. For me as soon as you mix the hardener you have about 30 seconds to use it. Dries rock hard greenish colour. Bit like snot! A pain to sand and seems to shrink leaving hairline craters at the edges which then need filler :reyes:.

I like the way you think. A couple of minutes spreading filler and then a good cuppa :tophat:. Jolly good. Funny, I have Poms and Yanks giving advice to an Aussie about products which may not exist here :rolleyes:.

Today I finished cutting shadows and new baseboard. Have shadows mounted ready to start planking. Tomorrow off to get 3mm balsa and search for Polyester Filler, Bondo, Isopol…

Will post report on what I find.

Larry L.