People buy fibreglass styling items every day, but from the questions that I get asked are always a little intrigued by the process of making the fibreglass and of making the moulds from which the fibreglass items are made.

I make moulds all the time, and this is a photo history of one I made recently. I have well over 500 moulds (total value just over 1 million pounds) and have been making them in numbers for the last ten years, and in total for around 20 years. That makes about 1 per week.

The process is quite time consuming, but I have developed a number of methods that I use on every mould, and this is an in depth record of making just one.

This particular spoiler is for the classic Mini. It’s not the first we have made, but we had a little accident with the last one - it involved a teleporter (nothing to do with Star Trek), which weighs about 6 tonnes, and accidentally drove over it when it fell off the loading pallet!! Not much left. I would not have minded so much had it been an older mould, but it was a new one, yet to recover its development costs.

Luckily we had a copy out of the mould, which was painted bright green (we use this colour a lot), and prepared for moulding.

The Master.

The first stage in making any mould is to make the master item. This is also often called the “plug”. This item must be very good. It needs to be thicker, and more dimensionally stable (does not flex) than real items, and prepared to a very high standard - every dip, or scratch on this item will be present on every copy made from the mould.

Thus its worth taking a little extra time to get it right - dips, craters, and ripples are the worst thing to prepare on fibreglass which means they are the most important thing to get right on the master, or you will end up doing the preparation on every copy - if you do it at this stage for the most part you need to do it only once.

Master is prepared and filled, primed and painted in 2k paint, which can be baked if you want.

Whilst it is possible to start making the mould the next day, experience has shown that releasing problems are certain if the solvents from the paint have not been given time to be released fully. It is best to wait a week, or two, and ideal to wait a month.

I tend to have a waiting list of items, and its normally not a problem for plugs to be prepared and then wait a month or more before being moulded. This definitely helps with release and shape of the item.


In order for the master to be released from the mould after construction, it must not have the fibreglass stick to it. If it sticks in the mould it can be so hard to remove that the entire mould is destroyed in the process, and you have to start again - don’t ask me how I know but it relates to letting a promising young trainee prepare a Cosworth mould some years ago!! I only do it myself now.

There are a couple of products on the market that seem to dominate for releasing, one is mirror glaze, which is excellent and we used for many years when we first started, and the other is honey wax, which does not go so far per tin, but being softer is easier to apply, and better in cold conditions. I use Honey Wax only now.

The idea of the wax is to make a very very thin barrier layer between the painted master and fibreglass of the mould. 

This is built up in a rather boring but essential process. Its a little like repeatedly waxing your car - both mirror glaze and honey wax make the very best car waxes on the market. You apply the wax liberally, and evenly over every millimeter of the surface, working it well into the surface. Then wait 5-10 minutes, before removing it. We use two cloths, as always with waxing - one to remove the wax, and one to buff it up to a high shine (second one is cleaner).

Its important to remove all wax with the cloths, as residue of wax will actually melt when the fibreglass heats up and reduce the ability of the item to release. Odd but true.

We apply the wax ten times, wax on, wax off, wax on, wax off, wax on, wax off, wax on, wax off, wax on, wax off, wax on, wax off, wax on, wax off, wax on, wax off, wax on, wax off, wax on, wax off, and by then I am always eagre to start.


For a mould to last for any number of copies it needs flanges around the outside. These flanges are essential if you hope to make more than a dozen copies or so, or if you want to make any quality copies. Only our in-house bodyshop one-off moulds are made without flanges, as everything else needs to sell in the hundreds to be financially viable.

Animal spoiler for Classic Mini being moulded

Autofashion animal spoiler mould being flanged

One-offs are made with two copies in mind, an original for the customer and a space for if it gets damaged.

Production bumpers, must be designed to sell at least 100 copies to break-even, more if we want to make a profit. Most moulds will stand between 200 and 500 copies before they need replacing and our Classic Mini Majic moulds have in fact normally done in excess of 1000 copies per mould set, before having to be replaced, with major refurbishing every 250-300 copies.

Tools required to flange finreglass mould

Flanging the mould takes quite some time to do, but makes the moulds so much better that it’s worth all the effort. It actually takes longer to flange the moulds than it does to build up the fibreglass layers!

The flanging actually carries out various functions.

  1. Firstly it makes the entire mould stronger, being at right angles to the edge of the component.
  2. It also reduces the natural characteristic of fibreglass to shrink, which causes a large sag over straight flat areas (most often seen as a sag between he headlights of a front bumper).
  3. It provides somewhere to release the component from.
  4. It makes it easier to “oversize” the component. This is what we do on most bumpers - all the edges that are critical to fit are made a little oversize, so that the customer can trim them back for a perfect fit - its much easier for a customer to trim a little off than to add some if he has a gap. All cars are different, and anyone telling you the fibreglass they supply is cut for your car, is lying. Unless they have had your car in their workshop how do they know what shape or size it is - it will not be the same as another car of the same type I can assure you.

Over the years I have tried a number of materials for flanging. Firstly I tried hardboard, and then formica, steel, and fibreglass, but the best material I have found in 3 inch wide strips of aluminium of around 2mm thick. Mostly its re-usable, and the properties of this material cannot be beaten by anything I have tried to date.

fibre glass mold being made

Aluminium is soft enough to be easily cut with tin snips, and to bend by hand to shape. Its solid enough to maintain its shape, and fixes well using hot melt glue, and withstands the heat of making the mould superbly, by actually venting the heat.

Using a hot glue gun the strips of aluminium are glues around the bumper, or spoiler being moulded. On a spoiler the position must be chosen very carefully as the flange forms the joint line between the two (or more) parts of the mould. On a bumper, or skirt its a little less important, but needs to be on the edge and at 90 degree to the egde of the component.

fiberglass mold flanged and ready for plastercine

molding a fiber glass mould in GRP for spoiler for Mini

I work a little at a time, and brace the flanges with aluminium angles. These help to keep it on. It only has to stay on until the first layer of fibreglass it on, and one I start flanging, the mould becomes delicate, and hard to store or move, so I tend to work until its gelled and got its first layer on, after which these are no longer issues.

Once flanges have been fixed around the perimeter of the item in question, the sharp edges must be softened with plasticine, rounded using a radius ball (8mm ring spanner will do if you don’t have one).

tools required to plastercine mould

fibreglass mould being plastercined


If two parts of the mould must be unbolted, then you must also fix alignment balls. For these roll a lump of plasticine into a ball about the size of a golf ball or ping pong ball, and cut it in half. Apply each half to the flange - no closer than 10-15mm to the component, and softened the edges with your fingers so the half-ball flows into the flanges.

You only need two alignment balls in theory for the second part of the mould to only be able to fit in one position, but again experience shows that fibreglass shrinks, and wears, and so I would use 4 or 5 on an average bumper, and 8-10 on a spoiler.

master item has plastercine and is ready for gel

This mould is ready to manufacture

Close up of location ball to align two parts of mould

Final preparation, is to double check that everything is still stuck down, securely, edges are soft, and location balls have not been forgotten.

A final re-waxing is needed as much to clean everything as anything else, but once complete you are ready.


The first layer is vital to a good mould. There are rules about making fibreglass that ensure a good bond between layers, and reduce the rate of shrinkage - all fibreglass shrinks (its the reason copies of fibreglass products rarely fit), but you need to keep it to a minimum.

Firstly the gel and resin first stage must be done the same day - you cannot leave it overnight, before doing the first layer of fibreglass.

The gel is still very sticky and very smelley - don't touch

The Gel is smooooth and we are getting somewhere with this mould at last

Secondly the Gel must be catalysed at the correct rate - too hot and the mould surface will suffer heat distortion, and will be brittle and chip a lot. To slow and it will take ages to go off or may not cure consistently. The temperate plays an important part, but also so does weighing the components. The manufacturers spend million measuring the products that they put in the fibreglass materials, if it was OK to guess they would too - it isn’t, and they don’t. fibreglass should always be mixed with scales, and with an eye on the thermometer.

I use two layers of gel on the mould - too thick and again the mould becomes brittle, too thin and heat distortion, and ripples can become an issue. The correct thickness of gel would be about 500grams per meter square. Most bumpers tend to be in the order of 2 square meters, so 1Kg of gel is about right for most.

You can just about make out the thicker gel.

Apply layer one - smoothness of the gel is essential if you are to avoid runs and sags which make the laminating so much harder than it needs to be. Lay down the gel - get it out of the bucket as quickly as possible, as it will go off far quicker in the bucket than on the mould. Paint the gel on, and then “lay the strokes off”, by brushing across your first strokes - this will ensure even coverage, and smooth finish. Play keen attention to corners where the gel may accumulate, or run.

The gel has gone off correctly when IT IS STILL STICKY, but it does not come off on your finger, if you run it down the gel. Most people think that the gel has gone off when it has stopped being sticky - if this happens it has gone too far. This is most likely to happen if you leave the gel until the next day - gel should never be left overnight, and normally will never need to be left more than about 2-3 hours to cure.

It needs to be sticky to ensure that you get good adhesion to the next layer. Once ready apply the next layer of gel in exactly the same way, and with the same notes. You can drop the catalyst down a little, as fibreglass cures by evolving styrene, and the second layer always goes off faster, as the first layer is slightly warmer and evolving styrene.


You need to cut the mat and get it ready - you want quite a slow mix of resin to allow you plenty of time to roll it very thoroughly to ensure no air whatsoever is left. Once the second layer of gel has gone off, and is within its adhesion window liberally apply resin all over the gel, and then mat. Wet out the mat and the roll in the normal way. I use 1oz mat for the first layer (300gram).

The first layers of laminated fibreglass mat and resin make this start to look like a mould

First layer of Mat on the new Mould

closeup of fibreglass mat - still wet

You should not be frightened to use a larger ratio of resin than normally as this will almost certainly be needed in order to get the air out and cover the sharp corners. The ideal strength/weight compromise of Fibreglass tends to be around 2:1 or 2.5:1 -i.e 2.5 time the weight of resin to the weight of mat. The first layer on a mould might go to 4:1 or 5:1 but the higher the resin content then the more liable to chipping the mould will be.

This layer should not get too hot, and should be left at least a day before the next layer is applied. Several days or a week is actually better, but it depends on your time scale.

Once cured fully, then the entire surface needs very thorough sanding. If fibreglass resin/mat is left for more than 1 hour a wax film build up on top, which further fibreglass will not bond to correctly, so it needs savage sanding to enable the next layer to adhere correctly. We use 40 grit sandpaper, and sand thoroughly. Clean and check for missed patches.

Sand the cured laminate with 40 grit

It is important to sand thoroughly, and not be tempted to skimp. This again is a time consuming process, but if the different layers do not adhere to one another well then the mould will crack very quickly, which will then be covered in spider (impressions of cracks) very quickly. Spiders are inevitable, but you can put them off by making the mould well. Spiders get worse and worse and eventually the section of the mould will breakup and come out - needing extensive repairs.

The next stage would be to apply the first double layer. I would use two layers of 1.5oz mat (2 x 450gram). Again it is significant to get the major air out, although minute particles of air are no longer a concern. Heat is. If the mould heats up too much then the  dreaded heat distortion will become a major problem. 

Then it starts again with more fibreglass laminate

The corners now start to look at little more rounded as the layers build up

An extreme close up of the fiber glass

The next step is a repeat of the steps above - wait for it to cure - at least next day, possibly next week, or even best a few weeks. Then apply another double layer, then repeat again with a third double layer. This provides a mould that is 10.5 oz thick, although actual physical thickness will depend on resin ration, and partly shape, but it should be around 10mm-12mm thick.

Third layer and edges look a little furry

It is a good idea to brace or put bench rests on the reverse of the mould.

Bracing - I would use 2inch x 2 inch wood to brace the mould and prevent it from twisting, or distorting, but the issues are that it makes the moulds heavier to move, more difficult to store, and takes longer to make. In an ideal world all moulds would be fully braced, however in the real world that is not always possible, and most of my moulds are not extensively braced.

Beach Rests - this is again 2 x 2 wood, and does brace the mould a little, although the purpose is to allow the mould to sit flat on a bench.

The bracing and bech rest in one hit

In many cases the bracing forms both functions - that is certainly the case here.

The wood is glued to the sanded mould using filler, and then laminated over.


Once all this has been done, and the fibreglass has fully cured (next day if its an emergency - longer if possible) then you can remove and release.

In the case of a spoiler we are only half way there and its only the flanges and plasticine that will be removed. If its a full bumper, most can be removed (if you wish), and the returns (if there are any) be attended to by repeating most of the above.

Those flanges now have to come off - after all that effort

After which is starts to look a little more designed, and less happened!

Much nicer looking fiberglass mold

Nice Animal spoiler mould - half of it anyway

Since we are talking about a spoiler here, the mould is turned over and sits on its bracing, and the aluminium is then removed, as it the odd pieces of tape I applied to protect bits. Once there is none left, the wispy bits of the mould are trimmed away with a grinder, and diamond disc.

Once that is done it’s all starting to look like a mould. The plasticine for the location balls is dug out and the balls sanded (I use a dremel first then by hand) into a smoother surface with a softer transition between the plasticine and the flange.

This must then be waxed - the balls must be waxed ten times as per everything before, and then the entire thing another time.

You will need to add lever points to enable splitting of the sections of the mould every time a spoiler is made, without having to damage the mould. This is just in the form or raised pieces of plasticine around the flange, in a similar fashion to the location balls.

Start Again.

Now we have to start all the fibreglass again, working through all the stages again, and with the same dangers. If we try and hurry any stage the mould will probably be junk, so we don’t try to hurry. A good mould is better than a fast one.