Mixing Resin and Topcoat

Matting man mix resin

How to mix fibreglass Resin and Topcoat

Not only does this article show how to mix your roofing resin, but how to look after it, and fibreglass a roof in cold conditions like winter.

Calculate resin and top coat

Firstly you probably will have already noticed I refer to everything in Litres and square Metres, if you have read other fibreglassing sites you may have noticed they work in weight ( Kilos ). I work in Litres as it’s easier to estimate and to explain amounts and coverage.

fibreglass roof resin weight 2

Roof resin and topcoat weight – If purchasing a 20 Kilo keg or drum you can expect it to contain about 18 to 19 Litres. Pictured is the average CSM chopped stand mat weight even though it is sold in metres and weight.



Estimate how much roof resin per metre square of matting

450g fibreglass matting roll fibre glassLet’s assume you are using 450g ( 1.5oz ) chopped strand mat. If you allow 2 litres of resin per square metre you should find it easy to work with and gives a good dry spot and pin hole free coverage. So for a 10m2 roof with a 1 layer 450g laminate you would allow 20L of resin.


  • 300g CSM – 1L – 1.5L resin per m2
  • 450g CSM – 1.5l – 2L resin per m2
  • 600g CSM – 2L – 2.5L resin per m2

In warm temperatures or with a thinner grade of roofing resin you may get a better wet out and need slightly less resin than quoted. On large roofs get a quote from your supplier but remember to allow a little for wastage.

How much Topcoat per square metre

Fibreglass colour coat

Topcoating – This video on applying GRP topcoat is part 5 of 5, and shows the basics of how to work the colour coat around the surface of the roof. We also see final preparation, masking fascias, mixing, and application to details.


If the weather is warm the topcoat will be thinner and easier to spread, expect to work at about 1L of topcoat for every 2 square metres. In cold weather the topcoat becomes thicker and doesn’t flow as easily, so allow 1.5L per 2 square metres.

Resin storage

If the weather is very hot make sure you keep resin and topcoat in the shade at all times, likewise in very cold conditions it may pay to bring resins or topcoats into a gently heated environment the evening before any usage. Remember the resin is the lifeblood of your new roof, don’t abuse it. Never store resin or topcoat for longer than the manufacturers recommend as this may come back to haunt you, and do not buy more than you know you can use, this means you need to turn over your stock and keep it fresh.

Mixing fibreglass roof resin

Saftey glassesAlways wear a full wrap around eye protection. Seriously!


I like to use a 15L roller bucket marked inside in litres, or on small jobs a 5L paint scuttle again marked internally. Always measure the temperature of the roof decking before mixing any resin, and on a hot day keep all resins and topcoats in the shade.

Fibreglass resin catalyst

Add catalyst – Simply mix in the normal manner by measuring out the desired catalyst and pouring it onto the resin, but be careful as the catalyst can sit on the surface and be quite splashy until it gets under the top surface of the resin.


When adding Catalyst my preference is to use a 60 or 100ml syringe for adding catalyst as it injects the hardener underneath the surface of the resin or topcoat. This way it mixes faster and there is less splashing if the catalyst lays on top of the resin, as it normally does when you use a standard catalyst tool.

Never use less than 1% or more than 3 – 4% catalyst with resin and mix thoroughly.

Mixing roofing resin 2

Inject catalyst – Draw up the exact amount of required catalyst and inject it in a figure of eight motion from about 6 ” away (150mm ) the force of the stream will penetrate the surface of the resin easily.

See Catalyst chart further down


Your fibreglass supplier should send you either summer or winter catalyst depending on the time of year but you will have to specify if you need LPT ( Long Process Time ) for hot weather.

Mixing fibreglass roofing resin or topcoat

Temperature and weather are the arch enemies of the fibreglasser but lets us deal with weather first.

Rain Icon weatherRain :  Do not fibreglass if it’s going to rain, or you think it may rain. Fibreglass hates rain which is good when the roof is complete but your absolute worst nightmare with matting on the roof, exposed roof decking or halfway through applying the resin. If it rains at this stage you may have to have a little cry.

Fog Icon weatherFog or damp : This may coat your decking or roofing trims with moisture and cause de lamination.


Temperature : Obviously the weather will play a big role so please study the Catalyst and resin drying and curing times provided below. Temperature does not only mean ambient air temperature but the temperature of the decking and the resin itself as these will affect the curing times greatly. A laser thermometer is a great tool for measuring roof deck and resin temperature.

Very Cold Icon weatherVery Cold weather : Below 5C and down to 0c and just below. There are now several cold cure resins on the market for working in these temperature ranges but bear in mind fog or damp associated with cold weather can cause delamination.


Cold Icon weatherCold weather :  5C to 15C use a normal fast Catalyst or winter Catalyst, you can also possibly use a reaction accelerator like ‘ accelerator G ‘ with normal resin.


Normal Icon weatherGood weather :  15C to 28C standard Catalyst or summer Catalyst


Hot Icon weatherHot weather : 26C – 36C Ideally use LPT ( Long Process Time ) Catalyst or slow Catalyst and keep resins and topcoats cool before use. Top surface may stay tacky for 48 hours or more in temperatures above 28C, this can be an issue for working on the surface or debris and insects sticking to it.


Roofing Resin and Hardener Cure times

Always try to use any resin or topcoat within the first 3 – 6 months of purchase, don’t stock pile old fibreglass resins. Curing times may become unreliable as might the performance of the resin itself, try to keep it fresh. Do not use catalyst if its over 1 year old.

Please use the chart below as a guide to curing times for roof resins and top coats…

Fibreglass roof resin and topcoat catalyst hardener guide drying timesCatalyst Temperature & Ratio Chart – Roofing resin and topcoat curing and hardening times complete with suggested ratios for different temperatures.

Click to enlarge

How to apply Topcoat

Standard flat roofing top coat normally comes in grey, often two but sometimes three shades. It is slightly thinner and a little bit more elastic than a normal coloured polyester flowcoat for fast wet out of the roof surface. A full range of colours are available however…

Make sure you apply the top coat in a generous manner, preferably about 1mm or so in depth, skimping will make application longer and can create pin holes.

Correct coverage will be between roughly 2 square metres per Litre of Top coat.

Topcoat pic

Timing – As long as you are topcoating within 24 hours you will not need to sand the fibreglass laminate you have just laid, however you may have little snags you may wish to sand first with 40 to 60 grit abrasive paper. Do not tread dirt or other contaminants onto your nice clean roof, if you do however clean gently with Acetone and a clean cloth.


Your fibreglass supplier should send you either summer or winter catalyst depending on the time of year but you will have to specify if you need LPT for hot weather.

Fibreglass and Grey topcoat seem to be the most popular choices with my customers, either the light grey topcoat which gives a look of old seasoned lead, or the dark grey topcoat that resembles new bossed lead.

Fibre Glass Colour ChartCatalyst ratios – Never use less than 2% Catalyst or more than 4% with Top Coat and mix thoroughly.

A full range of colours are available.


Fibreglass roofing in cold weather

Yes, you can fibreglass in cold weather, but curing times with standard resin can be increased to a point where it becomes impractical without supplying heat. Much better to use a cold cure resin like the ones mentioned below, however cold weather can also bring other factors that you need to consider. One such factor is a cold and damp day and these should be avoided as the moisture can compromise the following…

  • OSB decking – Both storage and usage on the day, don’t let it become damp
  • CSM matting – Keep it dry in storage, before and during lamination
  • Trims – These can also quickly attract minute damp particles from the atmosphere
  • Topcoating – A fibreglass deck can easily become damp, topcoats will not only be ruined but hard work to correct afterwards

Cold cure resin and topcoat

There are now several cold weather resins and topcoats available, some of the coldest days can also be dry, so fibreglassing is still feasible. Manufacturers and suppliers have traditionally always been cautious about what to recommend with fibreglass, quite rightly they would much rather over specify and play it safe. However as fibreglass roofing becomes more mainstream, the marketing opportunity that is cold weather or winter resin, has lured them out into the cold.

Cold Cure Resin polyester fibreglass

Cold weather fibreglass resin can make a hell of a difference in colder climates or during winter months. See below for cold cure resin drying times.

Normal fibreglass resin will cure in cold conditions but very slowly, possibly many days, which makes it impossible to work with unless dry heat is applied form either an electric hot air blower, or an infra red heat lamp.


Fibreglass in cold weather – cold weather resin cure times

What curing time can you expect with winter resin?

When fibreglassing in cold weather, at  00 ( 32 degrees Fahrenheit ) and using 3% winter Catalyst using a cold cure resin, somewhere between 45min to an hour out of direct sunlight.

Obviously this will depend on wind chill, heat from below, sunlight and cold cure fibreglass resin manufacturer, but the drying times stated are about right for branded cold cure, and homemade cold cure resin.

How to make cold cure resin or winter resin

Firstly use whatever brand of standard roofing resin you prefer…

Cold Cure Resin Accelerator G

Add accelerant – Pour out as much resin as you want to work with in the normal manner and add some ‘ accelerator G ‘ The normal ratio is between 0.5 and 2% but check your suppliers instructions. Now add this to the resin but keep your catalyst well away for the moment. Stir in the accelerator thoroughly.


The chances are your resin will already have an accelerator in so use as little accelerator as possible to get the reaction you require, you will be accelerating an already accelerated resin. Obviously I do not know your supplier or their resin so I cannot sate figures.

Only now can you add the Catalyst, this should be the normal fast catalyst or winter grade catalyst. You may notice the mixture bubbling and in an exited state, I allow this reaction to calm for a few moments before applying to decking and CSM. You should get 20 – 30 minutes of usable working time depending on temperature and ratios, very comparable to off the shelf cold cure resins.

How to make winter resin cold cure resinWarning!Never put catalyst in before accelerator G and never mix catalyst and accelerator G together. Doing so can lead to combustion, fires or chemical explosion. Always do a small pot test first and apply to bandages or small test areas, remember your standard resin will probably already have a small amount of accelerant in.



And that’s how to make your own cold cure resin… Ideal for small jobs or fibreglass roof repairs in cold weather. Making your own cold cure winter resin is cheap and easy but it will void any resin guarantees if you are bothered about such things.

This guide can also be used to make cold cure topcoat ( low temperature topcoat )

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