I am designing a sculpture based on a Gothic window which will consist of a steel frame with 3 inset panels in cast resin. The panels will be the 3 larger interior ares in the drawing with detail on both faces. Each face will be moulded separately and then joined in a 'clamshell' type arrangement with the gap filled with rigid foam (possible with additional internal reinforcement).

The panels will have a lot of surface detail (based on a pattern made by primary school children) and the exterior surface will be metal filled resin (probably aluminium) and approximately 20mm thick, backed internally with several laminations of glass mat.

These panels will be bolted to the steel structure via weld brackets. They are not inherently structural but will be subject to light loads form flexing of the main structure plus wind loads etc.

In the past I have used polyester resin for this application but had problems with casting defects largely caused by the high viscosity of the resin plus metal powder. I have had much better results with PU casting resin for smaller, well supported castings but I'm not sure how it would bear up under long term outdoor exposure.

So I am looking for nay thoughts on the relative merits of PU vs polyester for this application. The mouldings should have a life of at least 10 years outdoors in a temperate climate (UK).


  • $\begingroup$ Why limit yourself to just polyester or polyurethane? Epoxy resins are available in a wide range of viscosities and you can always customize the resins by adding UV inhibitors to the resin or get a resin with UV inhibitors already added. $\endgroup$ Commented Jun 23, 2016 at 17:17

1 Answer 1


Polyurethane is the better choice

Both polyurethane and polyester resins need UV inhibitors added to form UV-resistance. The polyurethane will hold up fairly well to UV with the metal filings, and can handle the molding defects better.

However, a composite of the two may be better

Polyester can be handled with styrene addition. This lowers the viscosity of the working laminate and allows for a longer cure time, to resolve any problems. Additionally, polyester and other epoxy resins bond well to steel when using a methacrylate based adhesive. It may be best to coat your foam with polyester first for structure, and bond it to the steel, then cover with the final layer of polyurethane and metal filings.


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