Hi Phill.
Try looking on the net for grp suppliers, such as CFS Fibreglass for instance. You need to figure out the approximate surface area of your repair to be able to guesstimate material quanties. Also it depends on the level of finish you want - slapped on bandage v invisable repair.
As a rule of thumb, for lay-up resin, you'll need 2.5 times the total weight of matt if you are using chopped strand matt (CSM) or 1.5 if using woven rovings (WR). Matt comes in grams per square metre. The heavier the matt the stiffer it is. A general multi purpose weight would be 450g to 600g which I think would suit a flat trailer side. Therefore if you have 1sqm of 600g csm then you will require (2.5 x 600) 1500g of resin. The catalyst is usually supplied with the resin and is added at a ratio of 1 to 2 percent by weight. ie 15g to 30g. Note that 1sqm of 600g WR would only need 900g of resin. For the glossy exterior you need gelcoat. You can get clear and add pigment or buy pre-pigmented. If you want to use the gelcoat like a paint then you have to add a wax solution or you can buy it premixed, called flowcoat. Gelcoat/flowcoat uses the same catalyst as lay-up resin and at 2 percent by weight. Generally, allow 300g of gelcoat per square metre per coat.
If it is a flat panel to repair then you are in luck. Taper the edges of the hole with a grinder/sander. Use some cheap formica faced board for a mould and use a PVA release agent and/or mould release wax to stop everything sticking. Attach the mould to the outside of the hole. Apply 2 coats of gelcoat from the inside on to the mould then lay up over that with alternating layers of csm and wr, starting with csm against the gelcoat and going over the tapered edges of the existing laminate. If you have already replaced the plywood on the inside then lay up the matt from the outside on to the new ply and then apply a thick coat of flowcoat. Once cured, wet and dry sand and polish.
The thing with this sort of work is that you have to buy consumerables. Some brushes, one or two consolidating rollers, clean mixing pots, scales, scissors, disposable gloves etc and a catalyst dispencer is really handy. Not to mention the sander, sanding pads, dust masks, acetone etc
Hope this helps, good luck!
