I use a penetrating sealer, so its easily applied and doesnt form a film on the surface. I have tried a few suppliers, on the smaller jobs i use wickes sealer, does a good job and is cheap.
Do you do alot of sealing? is it worth you buying 20l drums from a supplier? It can get expensive and having it sit there in the garage cause you dont use it much is pointless (as i found out)
Just google penetrating sealer, there are a few suppliers out there. Im sure someone will be along soon that will be able to recommend a good product. Try and go for a solvent based sealer, i found it works alot better than water based stuff. Solvent based is harder to work with though.
Matt