GRP is what they call it - Glass Reinforced Plastic. That is to say the glass (or carbon or aramid) fibres reinforce the plastic, the resin matrix. There are other substances we add to the resin matrix too. More often than not they're just fillers of one sort or another but they have their effect; there are handful that us modellers use. But what are they?
Chopped Strands
Usually glass fibres, you might see them listed as 3 or 6 mm (or even 0.2 mm), but they can also be chopped fibres of aramid (Kevlar) or carbon. Used as a general filler. The trouble is, glass is quite heavy compared to other substances available. Its main advantage is its cheapness. You would use chopped strand glass when making a fuselage mould for example but not when constructing a pair of wing fillets for a glider. Chopped strand (or chopped rovings in a special resin spray gun) are very often used with polyester resins for cheap structures.
Phenolic Microballoons
The heading microballoons is a generic term we use for microscopic hollow spheres. These spheres, typically 50 microns in diameter, are filled with air and actually displace a large amount of resin and because of their lightness are far superior to chopped strand glass fibre. Phenolic microballoons (~250 g/litre) are a specific type easily recognised by their reddish-brown colour; their softness makes them ideally suited for wood fillets, i.e. they sand like wood mixed in the correct proportion. These are not particularly waterproof so are no good for model boat hulls.
Glass Bubbles
Glass bubbles or microspheres (40-80 micron) as they are sometimes known are the glass variant of microballons. It is the light white powder we modellers often mean when we say microballoons. Being made of glass they are slightly lighter than phenolic (~230 g/litre) but a little harder to sand. they're cheap too - buy a 5 litre drum from Scott-Bader* for a fraction of the cost you'll pay in the model shop. Waterproof too. (*Once known as Strand Glass - check UK Yellow pages)
Plastic Bubbles
Same idea again but this time thermoplastic microballoons (50 microns) - SP call theirs 'Fairlite'. Even softer and lighter than glass or phenolic microballoons (75 g/litre) An even lighter filler material. Cherbourg Express (UK) do one too.
Cotton Flocks
Cotton flocks or microfibres are very fine wood or cellulose pulp used as a cheap filler. Like microballoons it stiffens up the resin but because it holds a lot of resin is especially suitable for wood fillets. Wood tends to draw in resin and a mix of flocks and resin can act as a reservoir of resin for the joint. Making a fillet round a wood joint from glass bubbles and resin is more likely to give you a dry joint.
Thixotropic Agent
If you ever did any moulding you will already have come across white flocculent silica powder. Sold under various names such as Aerosil, Cabosil, DT075 or Colloidal Silica this material simply prevents the resin from running away (it doesn't sag, as they say) from where you put it. Sure, you can make a stiff mixture from any filler but this thixotropic agent, as it's called, needs only be added in the smallest quantities (5%) to get its effect. It's principal advantage is that it doesn't reduce the strength of the resin as other fillers, it actually increases it. Use it on its own (in corners and edges in fuselage moulds, for example) or with any other of the fillers to get the desired effect.
Pigment
These are just colour pastes for resins. Since they are very strong colours very little needs to be used. It doesn't matter if you use them with polyester or epoxy resin. Watch out though, they get everywhere. Dark colours are particularly useful for the first (i.e. final) layer in mould making. Use black to see the mirror finish in you mould. I also have some fluorescent, powder, pigments and of course there are the flaked metallic powders - fine flakes of plastic or coloured aluminium powder.
Sand, Chalk, Talc
These are just cheap bulk fillers. If making a box mould it is perfectly acceptable to make a trowelling mix (8:1 w/w) of epoxy resin and silver sand for the bulk of the mould. Watch out for drilling though - sand/epoxy mixes can be impossible to drill. Talcum powder and chalk are softer alternatives naturally enough.
Metallic Powders
Usually aluminium powder but you can also get Brass, Bronze and Copper powders too. In the case of the latter these would be used simply to give a metallic look but aluminium added to resin for moulds has the advantage of conducting away excess heat from the curing process. Some special mould resins come with ready mixed with aluminium or steel filings.