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Epoxy application techniques: Detailed instructions are available from the suppliers of  the epoxy you are using, but here are some important general guides to the use of epoxy products: Mixing ratios have to be adhered to EXACLTY. Unlike polyester resin, which can set off with varying ratios of MEKP, epoxy requires for each part of base an exact proportion of hardener as prescribed by the supplier. The best method for measuring the small quantities as used in small boat projects is to use disposable syringes of 20 - 60cc size (60cc is about 2oz). In practice I find the 20cc size works fine for most work. For polyurethane paint the larger 60cc size works better. Unlike epoxy resin, paints such us epoxy based primers and polyurethane finish paints contain thinners, which will attack the syringes. They will only last for the day you do your painting. Use one syringe per component. After use keep them leaning in an empty tin or plastic container with the plunger pulled to the top. Make sure that each syringe is put into its own distinctive container so it is difficult to mix them up - the bottom of the containers will get coated in base or hardener! If you should use the 'wrong' syringe accidentally, you can still dispense out of it, but after that dispose of it and use a new one. Syringes kept like this will be good for at least the whole project. The scheme to use is to work out the total quantity of resin required and from the mixing ratio work out how many syringe loads of each component it takes to achieve it. Keep a piece of paper and pencil next to you so you can make a tick each time you pull a load as it is easy to loose count... Hint: During cold weather it helps to warm the base and hardner slightly in a bath of hot water. This will make dispensing easier as when temperatures are low, the viscosity increases. The hardener is usually of lower viscosity and may well not need to be warmed up even when it is fairly cold. Do not mix too large a batch, i.e. 250cc (8oz) is about the upper limit as this quantity generates enough heat to make it go off too fast. As a guide, an entire plywood sheet can be sealed on one side using just 180cc (6oz). This assumes that you use a squeegee and that it is the first coat. The second coat will be more like 90-100cc (3 - 3 1/2oz). Mixing: Use plastic tubs such as are used by the supermarkets for purchases of salads etc. and after dispensing the correct ratio of epoxy components mix very well with a wooden stick. Make sure to include the sides and bottom too - they will be coated with a single component during pouring/measuring out. Unlike polyester, epoxy won't set off so rapidly - but you must choose the correct resin - hardener combination. Depending on temperature, slow cure epoxy will give you up to an hour of potlife. So mix well as any poorly mixed portion of epoxy will be below spec and may lead to failure of joints. Health warning: Epoxies are harmful while in an uncured state. Use precautions such as wearing rubber gloves and working in a well-ventilated area while applying fresh resin, and dust masks while sanding. Epoxies have a full cure cycle of 7 days, even though they may have reached handling strength after 24 hrs. Sanding epoxy resin that has not fully cured exposes you to a health risk and in any case a filter mask should be worn while sanding. The manufacturer's recommendations regarding use and mixing ratios must be followed. However, a common sense approach is all that is required and will soon see you mixing and applying resin in a safe manner. Fillers: The basic resin is too thin for filling of gaps or making fillets (such as on inside corners) and has to be filled to make glues or pastes of suitable consistency. Aerosil is a good general purpose filler and can be bought from your epoxy supplier, 250g/8oz should be enough for dinghy projects. If added in small quantities it will make the resin thixotropic, i.e. it won't drain so readily off vertical surfaces. Adding still more Aerosil will make a good gluing mix. The best filler to use for the thicker pastes needed for fillets is wood dust or powder. You can add some Aerosil to the mix to make the paste smoother. Adding fillers to the low viscosity epoxy does not interfere with the adhesive properties of epoxy so long as the consistency does not exceed that of fresh spreadable peanut butter. Custom epoxy pastes/compounds These are available from your epoxy supplier for filling and fairing, but home-made pastes are easy to make and cost far less than custom pastes. If you want to make a fairing compound that sands easily use one to two parts of talcum powder (French chalk) to three parts of Aerosil (by volume). Experiment with the exact ratios. Talcum powder is however hygroscopic, so should only be used when it will be over-coated with standard pastes. Butt-joints and taped seams in general: Hint: To ensure a flush joint on the outside face of panels cut a 0.3-0.5mm (12-20 thou) recess half the width of the glass tape along all the edges that will be taped. Use two layers per side of 6oz (200g/m2), or one per side of 12oz (400g/m2) glass. The glass will bed down into the recess and this will make the joint virtually undetectable once filled and painted. For the butt joints cut strips 95 mm (3 3/4") wide, a little longer than the panel width - two strips per joint, one inside, and one outside. Assembly must be done on a flat board or table. Place a strip of waxed paper on the table underneath the butt joint. I found that baking paper performs better than plain wax paper. If you can get it, use a strip of Mylar (drawing) film. This gives a better finish, but best of all it is more transparent and you can work bubbles out with your fingers by smoothing them towards the edges. Always place the inside face of the panels downward. With the panels joined this way the recess on the outside of the panel faces up. I use a staple gun to tack the four corners of the wax paper down. Mix epoxy resin (+/-72 cc/meter [3/4 oz / foot of tape]) and stir well. Start applying resin at first to the cut edges of the ply parts to be jointed. This will avoid a dry butt joint. Then apply resin to the underside of the panels, either side of the butt joint, sufficiently wide so that when set onto the glass strip the wetted out area covers it completely. Put them aside ready for the next step. Paint epoxy on the wax paper and set the glass tape into it and spread more epoxy on to saturate it. With a squeegee work any air bubbles out of this laminate. Make sure there is enough epoxy to wet out all the glass. Place the two panel parts with the pre-wetted face down on the glass tape taking care to center it on the tape. I like doing a dry run and make some pencil marks to make sure. Butt panels together taking care the alignment is perfect. A panel pin in each panel will help with keeping the alignment perfect. (Put panel pins in the margin where the chines etc. are going to be glassed) Next paint an area either side of the upper face of the joint, sufficiently wide so that when the glass strip is applied over the joint it is fully inside the wetted out area. This is basically the whole recess width. Work the glass into the wet epoxy and paint more epoxy on to the glass till it is well wetted out and wait for the whiteness of the glass strands to disappear. Again use the squeegee to eliminate air-bubbles. Cover with a piece of baking paper placed on top of the wetted out glass. If you can get it use a strip of Mylar film. Start laying the Mylar or baking paper from one end of the joint, gently laying it down and smoothing it at the same time to avoid trapping air), then place a piece of melamine faced chipboard (or MDF board) on top of that. The melamine faced chipboard strip (about 150-200mm/6-8" wide) is weighted with whatever is to hand. I use old motor cycle and car batteries as they are of a nice block shape and are fairly heavy. The chipboard/MDF spreads the load of the weights and flattens the epoxy/glass tape and keeps the plywood parts flat too. The pressure squeezes out excess epoxy and so has a similar effect to vacuum bagging. In fact one could take this one step further by using peel-ply, bleeder film and breather material between epoxy and the Mylar film. After the epoxy has set and while it is not yet fully cured (after about 24-hrs) remove the weights etc. and scrape off any excess resin to level out irregularities. Sometimes the wax paper slips a bit and of course excess resin squeezes out from the glass. For scraping I use a blade out of one of my hand planes. Planer blades are ideal; they hold an edge for quite long and re-sharpen easily. Sharpen often as that makes work so much easier. Scraping removes the bulk of excess resin easily at this semi-soft stage and later when it has fully cured, one can finish-sand with a belt sander (lightly!) followed by an orbital sander. This is also the time to cut the overlapping excess of glass tape off with a Stanley knife. Pre-coating or sealing plywood: Once the hull panels or any larger plywood parts are made up, including butt jointing, it is best to seal the surfaces with two coats of epoxy resin, sanding between coats. It is much easier to do this while the panels are flat on the bench, than when they are formed into a hull. One thing you may want to experiment with: If you do your parts layout on a sheet of plywood, drawing every part that goes on it in pencil or marker pen you could seal the entire plywood sheet prior to cutting out the parts. The benefit of this method is that you do not have to be careful that you spill epoxy off the edges of the parts. At most you may find that one or two parts are near the plywood sheet's edge. Use a plastic bladed squeegee to spread the epoxy after pouring it straight from the mixing container on to the plywood. This is a much quicker method than brushing or rolling and uses less epoxy. Use a brush only for solid timber. Quantities are approximately 60-70cc/m2 (1 3/4oz/sqyd) for the first coat and 32cc/m2 (a little more than 1oz/sqyd) for the second coat. Updated: 05 June 2011 Back to Tips and Hints menu

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