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Help Page

GRP Fibreglass is relatively easy to use, there are a few indicators of incorrect use. This section is designed to help you troubleshoot your problems. Identify your problem below and rectify with the suggested information following the diagnosis.


Sticky Water Clear Polyesters


Sticky Polyester Water Clear castings are common. Mainly due to mis understandings of the resin system. First, a Water Clear Polyester is 100% incompatible with latex. It will not set whatever you do. We have tried waxes, acetates and all sorts of release agaent barriers and none work. Simply put, forget latex for Water Clears!


OK so you have an RTV Silicone Mould. The piece cures fine but the back where its filled is always tacky. Why? Well its to do with moisutre in the air. Gelcoat suffers the same issue, but is fixed with the addition of wax. Adding wax to a water clear will cloud it almost certainly, so thats not an option! addition


What works for many is the exclusion of air. This can be achieved with a waxed sheet of glass (for flat backed moulds) or the use of cling film on more complex shapes. You will find that with the correct addition ratios of catalyst, stickyness will be a thing of the past!


But wait! I do that and now the face of my cast where it meets the RTV is tacky? Why? Well an RTV will leech heat out from the reaction. Polyesters must reach a set temperature to cure and harden, its part of the cross linking of molecules and polymer chains. BUT if that temperature cannot be reached, neither can full cure!


As RTV sucks heat out of curing plastics, so this temperature is not reached and hence a sticky finish. This tends to happen more in winter than summer. How can you get around this? Well ensure your resin is at a warm room temperature, up to 30 celsius and that you mould is equally warm. Less heat will be lost to the RTV during cure and tackiness should dissappear as a result.



Bubbles in Resin Casts


Fine detail, small scale work is a nightmare for bubbles. It is easy enough to entrain air during mixing, its very easy to entrap it in corners and fine details during casting. So how can I reduce the prolem to a minimum?


First look at your mould. Does it look like it has crevices or overhangs that may trap air? Yes? Consider carefully how you will pour it. Overhangs tend to require moulds to be poured at an angle, corners and fine details sometimes just will not fill. A blunt cocktail stick run through thin channels and into corners may help to release trapped air. Do not fully fill the mould, just add a splash and move around the material before back filling.


Also a proven method is that of dusting the mould out with some Talc. This method is fine if you are post painting (in fact the surface talc can act as an extra key for paint) but is not always OK for items cast and used as is. A residue can be left behind. However, by introducing some talc into the mould, dusting out the excess with a brush or compressor and then filling will help. The capilliary action of the talc dust in the recesses and corners will help draw in resin!


Your final option on creating bubble free resin casts is to invest in a Pressure Casting System. Cast under pressure at 60 psi, you should find bubbles a become just fragment of your history!



Sticky Gelcoat Post Cure


Polyester Gelcoats are designed to be used in Tools or Moulds. They are not made to be used as a top coat right out of the can when making repairs or topcoating a flat roof. But why is my gel sticky? Polyester Gelcoats are moisture intolerant when curing. They will stay tacky for a long time after cure and may even need rubbing down to get rid of the tackiness.You need to exclude air during the cure to avoid this problem. Easy to do with a resin cast, its a sheet of glass, but what about 50 square metres of boat hull!!


How do I stop this happening in the case of a large area? Simple. Add wax solution to your Gelcoat at 2% by weight (20g per kilo of gel!) , thus modifying it into a "Flow Coat" or "Flo-Coat". During application and cure the wax in the Gelcoat works its way to the surface creating a barrier between air (and its moisture!)and the Gel allowing for a tack free set.


It is worth noting that the wax will still be there after the Flowcoat has set on the surface. If you are going to laminate over that or paint or antifoul, a gentle rub down with 1000 grit wet and dry to remove this layer is required. Otherwise the wax will act like a release agent and cause your paint/gel/antifoul to flake off.




Sticky Finishes : Epoxy Resin & Water Clear Epoxies


Epoxies are very tolerant of conditions and highly intolerant of mixing ratios. 99% of problems with Epoxies is down to weights and measures and MIXING.


Any material not mixed thoroughly will end up as a soft, sticky or tacky spot on the surface. If its inside, you wont notice it, though it may appear as an optical fault in water clear it will impart changes on mechanical strrength. The biggest schoolboy error is mixing ratio. It must be as exact as possible! If you are measuring sub 100g lots, 1g increment scales are not accurate enough, you need 0.1. We advise 0.1g scales for mixes of all products up to 500g.


At 500g you need 1g accuracy. But assuming you have your mix ratio correct, what else could be wrong? The method by which you mix! It is essential with close tolerance systems like epoxies to ensure that all material from the sides and bottom of the mixing pot are combined. Any that are not fully combined will result in the same problem as mix ratio, as essentially its the same problem! Empty the Part B into Part A when you start. Mix well. Now empty back to the Part B container and remix. This should ensure a good mix.


Failing these above, early demould can also be interpreted as a sticky finish, so, try leaving your next cast in a little longer (an hour, two or maybe overnight to rule it out).




Bubbles & Bubble Issues : Polyurethane Water Clear Resin


If you are seeing bubbles in cured PU Water Clear even though the mix poured into the mould has been de gassed or allowed to naturally degass then you most likely have a reaction occurring in the mould after you have poured the resin.


Water Clear Polyurethanes are cured by Isophorone Di-isocyanate and this is very moisture intolerant. The Part B Iso reacts with moisture to form Carbon Dioxide Gas and these are the bubbles you are seeing. This can happen in one of two ways:


1. The mouldfaces are damp or are very cold. Moisture is created as the resin warms up and begins to cure. That moisture poisons the Iso Part B causing


2. The mould is not compatible. Tin or Condensation Cure RTV does not work very well with PU Water Clear. Use an Addition Cure. It is possible to use a Tin Cure Rubber, but it is very hit and miss and they must be very well cured.


It is also possible that the Part B itself has become contaminated with moisture. Try using a dry gas cap on such intolerant systems like Poly Purge. A quick squirt after each use ensures a dry Part A and Part B!




Bubbles in Plaster Casts


Bubbles in plaster are hard to erradicate unless you vacuum the plaster before use. Plaster is made up with water. Water has a property called "Surface Tension" which is why bubble bath works! However in a casting arena it isn't going to be your friend.


Fine details in moulds are the worst for generating entrapments. A quick and easy way to help your moulding be less bubble prone is to use a little Dishwasher Finish Liquid in your plaster mixing water. Just a drop mind! This removes surface tension and helps with bubbles on the face dramatically.


The mould can also be pre wetted in water treated with Finishing Liquid as well, every little does really help! Do not be tempted to mix plaster with more water to improve viscosity, it will make for a weaker plaster and not help that much.


The last trick that can be used (if you need to after the above and 99% dont!) you could also try dusting the mould with a hydrophillic powder. Recent tests show using either the plaster powder or french chalc seem to give the best results.




Trouble Shooting GRP


Although GRP Fibreglass is relatively easy to use, there are a few indicators of incorrect use. This section is designed to help you troubleshoot your problems. Identify your problem below and rectify with the suggested information following the diagnosis.



Wrinkling, Rippling or Orange Peel Effect

Often caused by under cure of the gelcoat layer. Check your catalyst addition level as well as your mixing efficiency. Avoid operating under damp or cold conditions.




Air trapped between gelcoat and laminate or contamination with water or solvent. Apply a generous layer of resin on the gelcoat layer and bring it up and through the mat. Make sure the mould, brushes and rollers are not contaminated.



Fish Eyes (Lace Curtain Effect)

The gelcoat is not wetting the mould surface properly. Avoid the use of waxes or polishes containing silicone.



Colour Streaking

Caused by pigment separating from the gelcoat or never properly mixed in the first place. Check pigment addition level, stir very well and consider spray application.



Fibre Pattern / Print Through

Gelcoat is too thin or under cured before beginning lamination. Allow it to reach full green cure before laminating, thicken the gelcoat layer and avoid damp cold conditions.




Air bubbles are trapped between the mould surface and gel layer. Ensure you don't beat air into your gel when mixing catalyst or pigment. Lay the gel with light even strokes and do not stipple. Consider spray application. This may also be caused by dust on mould surface. Ensure it's dust free using a tak rag or similar.



Flaking Gelcoat

There is poor adhesion between the gel and resin/mat laminate. Do not leave the gel too long before beginning lay up and certainly never more than 12 hours. It can also be caused by contamination of the gel layer before layup begins.



Star Cracks Hairline Cracks

Caused either by a sharp object impacting the gelcoat front side of the piece or a heavy blow to the reverse of the laminate structure. Be careful when demoulding and handling. Hairline cracks are caused by over needing the piece on demould. Apply even pressure at several points rather than pulling hard at a corner.



Distortion or Discolouration

Most likely the laminate has been exposed to excessive heat during cure. Adjust your catalyst (lower it!) or reduce the thickness of each layer of glass.




Caused by either contamination causing resin not to bond to gelcoat, or by a lack of resin. Can also be caused by a lack of wetting out of the mat itself. Ensure even and sufficient wetting out and roll carefully.



Tacky Finish

If the gelcoat is being used as a topcoat it should contain wax to stop cure inhibition by moisture in the air. If the reverse of the laminate is tacky try adding wax to the final resin coat on the back side.




If the cast deforms post demould try adding ribs or a resin rich layer to balance off the effect of the gelcoat. Also ensure that its cured sufficiently before demoulding.



Poor Hardness or Rigidity or Failure to Set in Whole or Part

Most likely is either catalyst not mixed fully or not at the right ratio or the glass is contaminated by water. Check your catalyst addition level (did you actually add it?), keep your glass dry, ensure the mould is dry and try not to lay up under cold and damp conditions.