Fiberglassing Plywood

On the JW Builders yahoo group there has been some discussion of late as to techniques for doing this. I have been glassing hulls for so long that I forget that others are a bit intimidated by it, and that there are ways of making a thorough mess of the job so thought perhaps (for those who don’t have a copy of the West System book on Wooden Boatbuilding or some similar publication) I could share my thoughts on how I do the job.

Now do note that there are probably as many “right”ways of doing this as there are experts to tell you how, and that this is only one of several approaches. But it works for me.

First, surface preparation. I have worked in plywood plants and seen what happens to the surface of plywood when its run through a huge sander late in the life of the sandpaper. Epoxy resin depends upon some degree of penetration for its bond, and if wood is heated enough by several hundred horsepower driving blunt sandpaper the lignin that welds the cells together can plasticise and seal over the openings in the cells and greatly reduce the strength of the joint that you are trying to achieve. That goes for structural joints as well as well as fibreglassing.

I run over everything to be ‘glassed with my random orbital and 80 grit sandpaper. Any bare epoxy from glue joints that are not sanded can have adhesion problems, traces of paint or filler, sealer or even nail and screw heads need a scuff with the sander to ensure a good take.

Next, I get my roll of glass cloth, and that may be 6 oz. conventional weave, 10 oz. biaxial, 10 oz. unidirectional or one of the “knitted” fabrics according to what I am trying to achieve, but they all go on the same way.

Note that fibreglass cloth intended for fiberglassing are sized or pre coated with a chemical specific to the resin, so check when you buy that you have the right stuff. Also, while some people have had success with polyester resins, some have had spectacular failures and I suggest that the odds are against you with anything other than a known brand epoxy resin designed for marine use.

I roll out a panel of ‘glass, cut it carefully to size and outline the area that it covers with a pencil line. The glass panel gets tagged with a bit of masking tape and the piece number on it. Fold it up carefully making sure that there is no sharp creases. Do the same until you have all the panels cut and tagged, the hull marked as to where they go and you are ready to go.

Most of the time I do not overlap the edges of the panels, it makes lumps that are hard to hide and it really needs a double layer with the joins staggered to do a top job.

You can overlap a joint between two panels along the centreline, or along a chine, but try very hard not to overlap crosswise as it makes a bump that can be difficult to sand out. I often tape the stem around the forefoot so when the boat is run up a beach or a launching ramp it does not wear through too quickly.

Before the micro balloon layer goes on I will fill with microballoon/epoxy paste the hollows around the edges of the bump that is formed by the two or more layers, and fair that separately .

With the first panel of ‘glass laid loose in place on the hull Mix about 100ml of epoxy in a shallow tray, and pour about 1/3 of it on a fairly flat area. Then gently spread it with a soft plastic squeegee about 150mm (6inches) wide. I bought several designed for scraping frost off car windscreens that are magic for this job (sorry you people in Surfers Paradise, you’ll have to find a substitute).

Work the resin away from the puddle, using enough pressure to ensure that there is only just enough resin there to properly wet the cloth.

For those who have not played this game before “wetted”fibreglass cloth goes semi transparent and with the right amount of resin has a slightly glossy appearance with a little fluid in the bottom of the weave, if it's “dry” it will look like damp sacking and as though its trying to return to its original white, and if you put too much resin on it's like paint runs. Be careful with the work, too much resin will puddle under the cloth and make big lumps, too little may mean that the cloth is not bonded to the underlying plywood.

Note that no matter how you do it you are going to have to fill the weave later on, so don’t try and do it all in one shot.

Note also that with some of the more porous plywoods the resin will sink back into the wood surface and it can require a second application to ensure proper wetting. Do this as you work on the first application, we are talking a few minutes here not hours.

I try to have a helper doing the resin mixing for me, so when I call for another mix its waiting for me just when I need it, both helper and worker need good protective clothing and the area needs really good ventilation as you have a very large surface exposed.

Working along the boat you will find that with care and practice you can squeegee the cloth onto vertical surfaces or even around an overhang, and by being organised with the layout of the cloths you can make really rapid progress.

Watch for dry spots (apply some more resin and work it in), places where there are bubbles under the surface, I cut a little slit with a razor knife and work the bubble out through that, and watch for areas with too much resin. Just chase that around with the squeegee until its spread out enough to not do any harm. All this does not need to take long, my last big job was a 24 ft power boat and it took about three hours from arriving at the prepared job and beginning to cut the cloths to leaving with the “glass all stuck in place.

Having the ‘glass all in place, and the epoxy cured do NOT be tempted to run your hand over it, there will be millions of tiny thread ends all stiffened by epoxy all standing up ready to slash your tender fingertips. I use an Auto Body file, the one with the flexible body and semicircular teeth (check out the 3M site) and run that lightly over the job on its flat to take the nap off. I prefer this to sandpaper but a light scuff with 80 grit will do the job if you don’t want to buy a special tool for a one off job.

Next job is a quick roller coat of straight resin, do this within 12 hours of the first application if you can.

If you are really organised a layer of “Peel ply“ applied over this can help to give a glossy finish but for a lot of people this is not easy, and on a compound curved boat there are some tricks that the average Joe wont be up with. So I carry on with a third roller coat using a blue foam roller as soon as the second one is tacked off enough to not be runny, and when that’s cured I lightly sand off using 80 grit paper. Do try not to get back to the fibreglass cloth as exposed weave can print through more so that usual.

Print through? Looks like sacking under paint!

Next, I take a shaggy roller and apply two coats of epoxy and microballoon mix, the consistency needs to be about like porridge, sloppy but fairly thick. If its applied too thin it wont build up the cover you need and too thick it sags. Note that the second coat goes on as soon as the first is well tacked off but not hard.

When that’s all set up, out comes the longboard. That’s a strip of plywood about 2ft 6in long and 4in wide, handles on each end and an 18in piece of 60 grit sandpaper glued to its centre section. Work this back and forth at 45 deg angles until the whole boat is matt finished, you’ll see any low places as the longboard won’t fit into the hollows and if you grind too much off the high spots you’ll get down to the fibreglass.

Now, if you are not too worried about a perfect finish, grab the random orbital with 100 grit sandpaper and sand off, don’t get back into the fibreglass, do leave all the weave well covered. Do wear a dust mask at all times!!!!!

If too irregular then apply another layer of microballons to the low area and start again.

It can take a while, but you will end up with a surface that is fair, the fibreglass weave well covered, and the microballons well prepared for paint.

On paint, I prefer straight enamel paints for most of my small boats, but in every case you will need an epoxy primer, then a high build undercoat, and then the finish coats.

Yes it can be brushed on, and here is how it goes.

Brush on two coats of lightly thinned undercoat, observe the recoating times on the can, and don’t be tempted to shortcut them.

Wet sand with a cork block and 220 grit wet and dry paper, be careful to check that it has no Zinc Stearate anti clog coating (in fact you should be careful to do this on all sandpapers, some paints wont stick evenly to a surface that has any trace of this) and ”flat” the surface until its even. If you have to, recoat and redo until you have it right.

Do the same with the high build undercoat, two coats then sand off, and the same with the first two coats of the finish coats but use 280 grit wet and dry for this. Any coat that you wear through will need you to start again, dont patch it, redo the whole side!

Note that the objective of all this sanding is to remove any trace of brush marks in each layer in turn, so you need to recoat and resand until you have the surface perfect at each stage, remember that its the last coat of finish coat is the one that puts the gloss on but imperfections in the very first coat will show up in the last one.

It’s a slow process and one that builds up the arm muscles (cheaper than a membership at the gym) but if you have good brush technique and have the paint thinned just right you will have a finish that will be at least as good a professional spray job.

John Welsford,

who does take some short cuts but still gets a good finish on those boats that matter.