From posting construction pictures of the things I have made, and am making, I have had a lot of requests about how I make a fibreglass mould from people that would like to have ago themselves.  While there is no set way of how to go about it, this means different people do it different ways, so this is my way.

This section shows how to make a mould in several parts, but itís not an ideal project to start with.  While it gives insight into what needs doing, I suggest you start with something far simpler to get the hang of things first, and its also best to try with something that has not taken you ages to make, just in case something goes wrong.

Please note: The production of fibreglass gives off fumes. Therefore always where a decent mask, with carbon filters to extract any vapours. Dust masks will not do, it needs to filter the air. Be prepared to spend around £25 on a semi decent mask, more if you can afford it.


So this is the dome of my R6 droid I am making.  Itís made from styrene which is not really strong enough and has too many bits stuck on to be of practical use.  So by making a fibreglass mould and casting a copy, it will be much stronger and in one piece.

When making a fibreglass mould, normally you would polish the master so the mould comes out really smooth and shiny, however with this being styrene, buffing and polishing are not a good idea, as it being fairly thin the heat generated by the buffing wheel will distort it.

So its cleaned up and sanded, then polished by hand with automotive cutting compound.  Next its time to mask off the areas for this section.  It having recesses means a one piece mould will never work.  So this will be in five sections, four sides and the top.  This will allow each recessed section to be separate so the mould can be pulled straight off. 

Then wax spray is applied, and this being an awkward shape to say the least, means I won't be polishing off the wax spray as you normally would.  I have applied several coats of spray wax letting each one dry between coats.  This is to make sure the mould comes off the original.  My main concern will be getting the fibreglass jacket off the the master, without damaging it at all, so it can be kept in case of an accident with the mould.

Once the wax has dried, its time to apply the first coat of gelcoat.  Now I find it best to use a colour not of what the item will be cast in.  So as this will be in white, I will use yellow.  I also tend to use different colours for different projects, so when stored you can see at a glance which is which.  My Biker Scout armour moulds are in red, while my Predator stuff in green and car related stuff in cream.

So to start with mix up the gelcoat and add the colour of your choice.  I tend to buy and use more white gelcoat so buy it premixed, but this results in pastel colours when the colour is added.  If you want bright definitive colours you need to use clear gelcoat as this lets the full colour show.

So first apply a layer of in this case yellow.  Make sure there are no gaps, and no brush marks.  Use a good quality brush with soft bristles, if harder bristles are used they can leave drag marks, which on a waxes surface turns into bare streaks.

The second coat is just white.  I find this lets me see where I have and haven't been making sure I cover all over, not leaving any gaps.  This coat as it has something sticky to stick to can be applied far quicker and also heavier in places that need it.

The third coat is back to yellow to cover the white, and again this can be applied fairly quickly. 

Make sure you cover any edges and recesses well, and build up areas that have sharp indents so they are smoother to the surface. 

You can just make out in the first yellow coat, the outline of the bit round the oblong recess.  As the second and third coat are added, this disappears meaning, its well covered.


Thing you will need, but bear in mind to start from scratch you are looking at spending around £100 for equipment and materials for.


Polyester resin, (Preferable Lloyds approved)

Grade 1 chopped strand matting, 300gsm.

Polyester gelcoat.

Colouring agent.

Large and small mixing jugs, and something to mix with.

Selection of different width cheap brushes as they are thrown away afterwards.

Consolidation roller, around 2 inches wide.  (This is optional but very useful.)

Pair of sharp scissors.

Disposable latex gloves.

Childrenís medicine syringe for accurate catalyst measuring.

Face mask with vapour filters fitted.

Can of aerosol mould release spray wax (not furniture polish.)

Short M6 nuts and bolts. (Not shown.)


Some old newspapers and paper towelling is also useful.


When it comes to laying up the matting, its best to remember that the matting needs to overlap each other well, if not done in one single piece.  While matting is lots of thin fibres held together with a binder, when itís cut that is the edge, and butting two pieces edge to edge will leave a weak line.  Each piece placed must over lap by a good few inches to insure even strength.

I start with cutting and laying up strips to go round the lip, and any recesses.  This makes sure you have a thicker edge than the rest, meaning when you come to prise the mould apart it wonít break or crack.

When the matting is wetted out, go over it with the consolidation roller to massage in the resin, and also push out any air bubbles.  Using a roller saves wasting lots of resin, but if you donít have one, just dab it with the brush, till it goes almost transparent.

I then cut larger pieces around six inches square and lay them on the outer lip wetting out towards the middle.  I work round the edge like this then place pieces to cover the remaining middle section.  On something like this mould, I will apply four layers like this of 300gsm matting.

When you have laid the matting up go over all of it with the roller to make sure the resin is well worked in and the air bubbles are out.

Finally you want to remove any access resin, and while some people use excessive resin, thinking it will make it stronger, fibreglasses strength comes from correctly proportional matting to resin ratios. 

The simplest way to get a good matting to resin ration, is dab the whole thing with the brush, wiping off any access resin the brush picks up.  Go over the whole things a couple of times, and you will be surprised at how much resin comes off.  When you have done that leave it for a few hours to cure well.

The next thing to do is sand the surface.  Why, well cured fibreglass strands are like needles and while most canít be seen, run your fingers across the surface and you will find them.  (Then have to pick them out with tweezers.)  So just get a piece of 100 grade sandpaper and go over the whole surface lightly, and this will remove them.  Once done, you can gently rub you hands over the surface and sand down any rough spots you come across.

While you donít have to do this, it makes handling the mould much less painful in the long run.


The next stage is to repeat the process on the other three sections of the dome.  In this case the moulds sides are in four sections, as there are four points on the mould that must be pulled off straight as there is a large or deep recesses.  The more or less of these determines how many sections the mould is in.  Also this mould has indent lines in the surface which although only go in one or two millimetres, they would cause enough of a problem if trying to pull the mould part in less than quarters.


So with the sides done, its time to do the top.  This requires all the tops of the sides and the top of the master to be well waxed and polished so the lid won't stick to any part of it.

When this is done, again apply three layers of gelcoat, then several layers of fibreglass matting, for strength.  This is left overnight to cure.

The next job to do is drill the bolt holes to hold it all together and this must always be done, before the mould is seperated to make sure all the sections are where they are intended to be.  If you separate them first, then try to drill them, things donít line up as well and gaps and misaligned edges are formed.

The next job is to carefully separate all the sides, clean up the edges and any marks that might be on the sides from blemishes in the master, which are quite common in styrene made things as marks are hard to get out well beforehand.  A useful hint here, is when in doubt about a mark, as long as its a dig or scratch on the master it will come out as a surface mark on the mould, which can then be sanded smooth.  If it is a hump or mark proud of the master this will cause a dent or recess in the mould which is much harder to get rid of. 

With the marks sanded smooth using a combination of 600, 800, 1200 and 2000 grade wet and dry abrasive paper, the next job is to polish and wax the mould.

This can take as long as making the mould itself, as if you skimp on the polishing stage, the first cast might stick, then all your hard work is ruined as a suck cast will ruin the mould trying to get it out.  Always remember, over waxing and polishing is better than skimping.

There you have in very basic terms I hope of how to make a multi part fibreglass mould.  Please remember if youíre going to have a go at this start with simple things, preferable for practice use things that donít matter if it goes wrong and the gelcoat sticks to it.

Donít use your pride and joy you have been working on for weeks, in case it goes pear shaped and it gets ruined.