Having spent many years working in the adhesive industry (not epoxies), I would like to warn against taking any actions such as thinning which are not specifically recommended by the manufacturer. If you thin an adhesive, in nearly all cases you weaken it.
Adhesive manufacturers of repute such as the WEST people employ skilled chemists to develop and produce their product. They have a far better feel for what works and what does not than any end user, especially casual home hobbyists. Use a thinner which is not appropriate to the adhesive and you run the risk of making a bad situation far, far worse.
WEST and System Three are two very high end epoxy manufacturers and you can (properly) use their product with confidence.
