Thank you everyone for all your tips, I have a better idea now what to watch out for, I had not heard of the 'window' expression, it is a bit of a mine field out there with so many different products to chose from and more coming onto the market all the time, some times the old tried and true methods just can't be beat and then along comes a new product which wipes the floor with the old ones, as is always the case it pays to experiment with them all and find the one which suits your particular project, thanks again all!
Gal