Thanks John, I'll have a look at their site although sods law says they'll be at the other end of the country.


Re:- P.S. why cant I quote from a previous page?

I think you need to, "cut and paste"

I hadn't thought of EPDM, but it might be ideal for the small porch I need to do before we sell this house (assuming we ever get the new build underway!).

The problem with some forms of GRP and moisture is to do with the type of resin used. Typical (i.e. cheap) polyester resin is slightly cure inhibited by moisture, plus its already relatively poor bonding qualities are reduced to near-zero if there's any damp about. This particularly applies to bonding to wood, where even a small amount of moisture can result in the bond being so poor as to be virtually non-existent. Using a relatively moisture tolerant resin, such as epoxy, for the base coat and lamination layer gets around the problem, but adds a lot to the cost (epoxy resin being maybe three to four times the price of polyester). Vinylester resin might be an in between option, as it generally tends to be more moisture tolerant than polyester and is only maybe 50% more expensive. One downside with using epoxy is that it needs a UV resistant top coat, as it's far more vulnerable to UV induced breakdown than polyester.

I'll disagree about epoxy, as I've been using it for many years making composite parts. In fact I'm part way through making a small epoxy/glass/carbon over foam core boat (out in the open) at the moment. Certainly it has a delayed cure in high moisture conditions and the resulting cured resin has poorer mechanical properties. It also needs to have excess surface amines that tend to form during a moist cure washed off before recoating, but bond strength remains much better than even polyester applied over very dry wood. Laminate strength isn't that critical for roofing, I'd have thought, especially as an epoxy/glass layup will be significantly stronger, and more moisture resistant when cured, than a polyester/glass one.

Polyurethanes are fine in the damp, as the majority are addition cured and absorb moisture as they cure. They are all probably too flexible when cured to be used in a glass laminate, though, and despite their good bonding qualities I've not seen any suitable for hand lamination with glass. A PU bonding sealant could be applied to a slightly damp substrate before applying polyester, I suppose, rather like the way you'd prime something that inhibits polyester cure, like concrete. The PU would bond exceptionally well to the wood (better than polyester) and I suspect the styrene in the polyester would ensure that there was a good chemical bond between the GRP layer and the PU primer.

OOPS that didnt work properly, but I guess you know what I mean

You can get pretty benign epoxy resin now, stuff like the Syntac resin from your neck of the woods (and which I've been using recently) is non-toxic. All epoxy resins are bloody expensive though, even the Syntac (which is relatively cheap) is around £13/kg in small (less than 10 kg) quantities.

Same problem. It is not so much the sheet absorbing moisture that causes a problem, it is the moisture. It is a matter of planning it for a time with no rain or dew forming, but not a day when it is really sunny as that can cause an uneven or very rapid cure.

Best not to do GRP work in the dark

The main point is that things have to be dry to make a good job of it.
UV speeds up the cure but tends to cure the exposed layer, this can slow the cure underneath because of the trapped styrene fumes. Some resins are better than others in this respects, I never had any problems with Cray Valley Encore 30, but one of the Norpols (think it may have been 420) was very bad. Things have moved on since I got our of the industry though.