SIG do a thinner. For brushing go 50/50 on first coat then about 30% thinner on the second.

If you are talking about he small scale models that Guillow make, those that come out at about 24 inch span, then you will be adding very significant amounts of weight when brush painting. If you are planning on fly them then the performance will be very significantly degraded. In fact they may not fly at all.

If you are planning on flying them then use colored tissue and the absolute minimum amount of dope on the tissue. See here for a guide on how to do this. http://www.ffscale.co.uk/comp7.htm

I have been a modeler all my life, getting these small scale models to fly at all is a challenge to fly well is really really difficult.

The 1/4 scale Camel is not so sensitive to weight but keep the rear end light at all costs otherwise you will be adding pounds of lead to the nose to get the CG right..