Two brushed coats are done at first with the second coat following within a few minutes of the first. Most resins will bond chemically within 24 hours without sanding but sometimes get an amine blush or B reaction on the surface which prevents the resin coats making contact with each other. The resin I'm using at present does this quite quickly so I do the second coat almost immediately, this works out well as surprisingly the second coat seems to go on top of the first even while the first is still runny.
Once the first double coat has gone off overnight I sand it with 40 or 60 grit and apply another double coat. The sanding gets rid of any blush, basically all shininess has to be removed.
All going well the third coat is a single one ( a single coat is smoother ) but if necessary i.e. if there is any sand through or more fairing is needed a 4th coat single can be done.
The board should be turned over as soon as the first side has set enough to allow it, after the rails have been sanded and a new masking tape skirt has been applied.
The same process is repeated on the other side.
With each coat it's a good idea to spot fill any imperfections when the coat has gone semi hard but still tacky. This can be done with left over tack resin or resin with sone sanding dust added. Rather than trying to smooth the resin on to the spot being filled it's often better to just dab it on so that it sticks up and doesn't spread out. The protruding bit will sand off more easily that way. Of course if there's a gap which goes into the board deeply one should push the repair glop in as far as possible.
Once all the coats are done I usually leave the board for a week or so to cure. That makes sure that all the coats are of equal hardness, otherwise the top coats can be softer and sand off too easily. With hard resin coarser wet and dry grit should be used for longer than with softer recently cured resin, otherwise if the grit is too fine at first one ends up polishing the tops of bumps and into hollows rather than fairing them.
Natural bristle brushes are best, freezing the brushes can allow them to be re used. We don't use any solvent for cleaning up, it's easier to just avoid making a mess.
For a perfect finish several micro thin coats can be done with a foam roller, only 50 to 100mls is needed per coat, with fine wet and dry sanding between coats.
Some resins are designed specifically for getting a build up, but I just use a standard laminating resin. Glass cloth is used as a resin dam preventing resin running off rails and as fairing compound which makes things easier but itchier.
A pilot hole where the vent is going to go is a good idea too, to prevent pressure buildup inside the board as the resin heats up during cooling, this can force air out through the resin making pinholes and potential leaks later.
