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Simulated Metal Surfaces (Cold Cast Metal)

Polyester resins and finely ground metal powders can be combined to produce very realistic simulations of metal castings. The basic requirements for successful casting are extremely simple but tend more to be an artistic process rather than an engineering discipline.
The best, most realistic effects are obtained by mixing as much metal powder as possible into a gelcoat/resin mixture but from an economics point of view the normal mixture for copper based metals is 3 parts metal to 1 part resin by weight.

Moulds

Many castings will have undercuts and therefore require the use of flexible moulds which can be made from silicone rubber, latex rubber or PVC compounds, while rigid GRP moulds can be used if undercuts are not present.

A Suggested Casting Procedure

1. Calculate quantities required. Allow 600-1000 grams gelcoat/resin per M2 and allow 1800-3000 grams metal per M2

2. The viscosity of the gelcoat determines the amount of metal that can be combined with it and the ease with which the gelcoat/metal mixture can be brushed into the mould surface detail. A good compromise is to make a gelcoat/resin mixture by combining equal parts of gelcoat and non-waxed laminating resin, e.g.

The mixture should be applied to the mould surface by brush taking care to thoroughly brushout and stipple into any surface detail. The gelcoat/metal mixture should be left for 30-90 minutes to cure.


3. The cured gelcoat should be reinforced either by a resin/filler mixture or a glassfibre laminate.

4. When the casting/laminate is fully cured and removed from the mould it will require polishing by firstly rubbing gently with fine (00000 grade) steel wool to remove the outer skin of resin and then buffing with fine metal polish.

5. If a 'bright' finish is required the exposed metal surface should be wax polished.

6. If an oxidised or verdigris finish is required it can be obtained by an acid treatment: place the casting in the open air, cover with a polythene tent and place a saucer of sulphuric acid beneath the cover.

Gelcoat Repairs

Unfortunately even the best quality G.R.R moulding will occasionally suffer from a hole in the gelcoat where the reinforcement was incorrectly applied.
These holes are very difficult and probably impossible to repair so that they are completely invisible, but we feel that the following notes will enable the best possible result to be obtained.

1. Carefully remove all loose gelcoat with sharp scraper or pricker.

2. Wash out the hole with a perfectly clean brush wetted with perfectly clean acetone. DO NOT subsequently touch the hole as natural oils from the skin will cause a black ring to appear around the edge of the hole.

3. Make up a gelcoat mixture using exactly the same pigment and catalyst percentages as was used for the original gelcoat. To simplify accurate pigment and catalyst % addition we suggest a mix of at least lOOgrms.
N.B.  If just a small amount of gelcoat is used and catalyst added by guesswork the probable over- catalysing will result in a green tinge.

4. Do not put the gelcoat mixture straight into the hole to be repaired as small pinholes will result -brush out the gelcoat on a sheet of polythene and rebrush at 90°to the first strokes and then scrape up the brushed out gelcoat and apply it to the repair.

5. Slightly overfill the hole, allow the gel to fully cure and then carefully rub down and repolish.