Frequently Asked Questions
about Fiberglass Diy Repair

Diy repair home

Here we will have some of the most frequently and not so F.A.Q about fiberglass diy repair and the associated products that I get from people when they are complementing doing their own work or when something has gone wrong. Along with this type of structure, there are so many different types of fiberglass resins, paints, core materials etc. for different applications that we are lucky that any one product has the ability to be used for many different types of applications.

If something does go wrong with our diy repair or project or did not turn out the way it should have, most of the time the problem can be traced back to us even the seasoned professional fibreglasser can make mistakes, sometimes though the weather, the place of work or the product itself can be the problem.

With this in mind FAQ has been made to hopefully solve many questions that have been asked about fiberglass repair and doing it your self. As time goes on I will continue to add to this page.

As more and more people want to do their own work or they just want to know something about what happened to their diy repair and if those questions / answers are not on here then there will be a form available that you can partake and send me your question and I can email your answer back to you or point you in the right direction for you answer.


Why are some resins called waxed or un-waxed?

Why has my stored resin got jelly lumps forming under the lid?
How long will stored resin keep for?
After I mixed up my resin it gelled sooner than expected,why?
When I mixed up my resin it changed colour, why?
My resin has a tacky surface, why?
The resin I just bought is thicker compared to the same resin
previously bought,why?

Why are some sections of my job not cured while other sections are?
Can I glass over waxed resin?

My stored fiberglass mat has gone stiff and prickly all over
or in some places, why has this happened?

How should I store my fibreglass mats?

What is "bog" and can I make it and where do I use it?

Why are some resins called waxed and un-waxed?
These terms usually talk about polyester resins, the first one waxed is a orthophthalic resin used in general fibreglass work where it is usually left a "natural" finish.This resin has a wax added to it to protect the resin
and underlying glass fibres from moisture penetration. The wax coming to the surface as the resin starts to cure (dry). for most diy repair projects rhe reain supplied in kits will be waxed resin.

The other resin which is called isophthalic resin is sold un-waxed, this resin is also a little bit more expensive to buy, is a higher quality resin being stronger in nature, has better chemical resistance
and is usually the preferred resin used in fibreglass boat building and repair work as it can be left for a day or two without sanding prior to further laminate build up.
Wax is best added to this resin on the last coat if it is being used under a boat floor/hull situation or left without surface finishing (e.g. painting) to protect from moisture entry.

Why has my stored resin got jelly lumps forming under the lid?
This is usually a sign that the resin is getting old and/or not properly stored and especially if you have used some out of the tin as air space inside the tin speeds up the process.
Resin in this condition needs to be used up as soon as possible or it will be beyond saving.

How long will stored resin keep for?
Polyester based resins usually have a shelf life between 6 to twelve months if not opened and stored in a cool, dry place. When buying resin, especially if you are not going to use it all up within a short period of time and your buying a fair amount try to buy from a major outlet that specializes in fibreglass materials (example- resin) as these guys are usually going through large quantities and so is bound to be fresh.

After I mixed up my resin it gelled sooner than expected,why?
There can be several factors why this happens, the most common will be when we are outdoors, it may have been overcast when we were using a mix, then when we go and mix up another batch (using the same amount of catalyst) the sun has came out (passing clouds) and caused the air temperature to rise around our work and also in our resin mix container making the resin gel faster than the previous mix.
Make sure as well that when mixing catalyst into resin that you don't take too long doing it but mix it well.
When fiberglassing outdoors it pays to keep an eye on weather formations so the resin can be adjusted slightly with the next batch if need be or shift your work to an area where the temperature is more stable.
You may have accidentally over catalyze the resin making the resin gel faster than it should.

When I mixed up my resin it changed colour, why?
This is a normal chemical reaction that takes place when you add catalyst (hardener) to most resins as it is an indicator to show that catalyst has been added after it has been mixed together changes the resin colour and character to allow the hardening process to take place.
Many resin manufacturers may/will have different colour resins to indicate the type and/or the resin manufacturer but when catalyse it all changes colour to indicate the reaction has taken place.

My resin has a tacky surface, why?
If you have been using isophthalic resin or gelcoat this is a characteristic of these products made by the manufacturer as this allows the resin or gelcoats to have more/added layers of reinforcements laminated over them without the need of "sanding" the surface.

The resin I just bought is thicker compared to the same resin previously bought,why?
This sometimes happens when resin that is stored has not been or has been forgotten to be rolled back and forth (if stored in 225 kg drums) prior to de-canting into smaller drums or tins.
Resin has a tendency (like many liquids) to "separate" or to settle as the styrene comes to the surface making the resin thinner on top and thicker towards the bottom. Before using resin from your drum or tin it is good practise to always roll or move back and forth the resin to mix before putting into a smaller container to catalyse and use. Resins also get thicker in cold weather and can be made thinner by placing the resin drum in the sun to warm, make sure the resin is moved back and forth to uniform mix the resin.
Resins that are thick in consistency will take longer to penetrate the fibres of your reinforcements (example-fibreglass mat).

Why are some sections of my job not cured while other sections are?
It is most important to mix your resin and catalyst (hardener) properly with a flat type mixing stick and working around the outside edge to the centre of the container and visa versa till you have a uniform mix, try not to use any fast drill on a paint mixer or egg beater as you do not want to introduce air bubbles to the resin mix as it will be harder to roll these out of your mat or reinforcements.
Do not use a round stirring stick as these will not mix the resin and hardener well enough for a uniform mix causing un_catalyized resin to enter your job.

Can I glass over waxed resin?
Generally speaking, no, if it is to be a repair or a continuation of a job that has been left with a coat of waxed resin for protection, all repairs or add-ons that have lapsed need to be sanded before new or more laminates are to be applied to insure a good bond.
If you fibreglass over such an area, like for instance the floor of a fiberglass boat the coating on the floor (especially older style boats or if the floor has been replaced) would normally be Flowcoat which is sold waxed or under the bonnet or inside the guard of a GRP(fibreglass) car or truck which has a waxed finish, if you glassed or fiberglassed over such an area the repair or material you put on would de-laminate and/or peel off the surface when dry or in the near future making such a repair useless that is why sanding/grinding the surface free of contaminated has to be done to insure a good bond.
But if you are glassing over a waxed surface that has been prepared to be used as a mold, then yes you can providing proper guidelines have been used in mold preparation.

My stored fiberglass mat has gone stiff and prickly all over or in some places, why has this happened?

For the diy repairer this can happen sometimes if your protective plastic wrap or bag has been left open exposing the mat to the elements like moisture or it has become damp or wet with water causing the binders that have been added to the mats to become "misplaced" or "lost" resulting in the mat losing its soft and pliable texture.
Mats in this condition are harder to wet out resulting in many fibres not being saturated properly resulting in a poor quality laminate panel that could easily fail under stress loads.
Affected mat may be found very difficult to roll over corners or rounded edges and may not lay properly resulting in air bubble formation at or near the corner or edges of your work.
Uses for this affected mat can be very limited, maybe if you own a boat for instance and you wanted to make a fillet/bait board then you could lay up flat sections of laminate and cut into appropriate sections, hold together with a bit of car bog/filler then glass up the joins with new mat, then finish off in a flowcoat and add a piece of cutting board to the bottom face and mount it in a spot on your boat.

How should I store my fibreglass mats?
Fibreglass mats should be rolled up prior to storage or putting away and some strips of masking tape or even packing tape applied in places to hold the mat from undoing then putting the (if you had purchased a full roll) mat inside the plastic bag and placing back inside the box. If on the other hand you have bought a few meters or so at least put a plastic shopping bag either end and tape it shut as this will help keep your mats in good condition for next time.
The thing is if you don't roll it and you say just fold it up like a flat square what will happen (especially to CSM-chopped stand mat) is that after a period of time when you go to unfold it to use it the strands of mat on the square corners tend to pull apart and stay raised making the mat look like tile formation, it will also tend to fall apart where you don't want it and can be harder to wet out at those points depending on how long it has be left folded. Rolled up mat on the other hand keeps its manufactured shape and constancy so when you unroll it, its just like when you bought it, make sure you store mats in a dry place.

What is "bog" and can I make it and where do I use it?
"Bog" is is a mixture of polyester resin and commercial talcum powder mixed together to form a smooth semi-thick paste, when cataylst (hardener) is added it sets to a very hard rigid dense compound. It will be found to be harder to sand compared to car fillers and is not flexible in nature.
It is easy to make, first you will need some polyester resin the amount will depend on how much bogging or filling you will be doing but generally speaking if you say you use a cup full (250mls) of resin it will not expand to much more as the talcum powder is added as like adding flour to water, in this case when you add talcum powder to the resin it will slowly get thicker till you reach your required thinness or thickness depending on what your doing with it, but generally speaking the usual consistency should be fairly thick and not fall off your stirring stick.
When adding catalyst you will need more than what you usually use for setting off resin as the talcum powder is now acting as a inhibitor so you will need to add 1.5-2.0 % catalyst or a margin more (depending) on your climate. Just like adding catalyst to resin it too needs to be well mixed especially around the outsides of the container, make sure you use a separate spatula if transferring bog from your main container to your mixing tin as any catalyzed bog entering your main container will eventually cause gel or hard lumping in the container if your not using it all up, try to make only enough that you think you can use. Best applied by spatula or putty knife depending application.
Where to use it It is most commonly used in the boat building and repair trades. When fiberglass boat manufacturers are joining decks to hulls it is used to fill in any un-evenness (air gaps) between the deck and hull and help bond the deck to the boat whilst they staple together to make weather proof. Is also used in filling the rubber gunnel for bonding prior to mounting to the hull/deck join.
When hull bearers are placed in the fiberglass hull it is used as a filling compound as well as a bonding agent prior to glassing the bearer/s to the hull, putting across the glassed bearers and deck supports before attaching the floor to the bearers and supports etc. If the diy repair does not require the bog to be sanded or coated over, add some wax to the mix (if your using unwaxed resin) as to stop moisture penetration to the bog.